Edgar Filing: MidWestOne Financial Group, Inc.

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ATBANCORP FINANCIAL STATEMENTS
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Filed Pursuant to Rule 424(b)(3)
Registration No. 333-227899

Dear Shareholders of MidWestOne Financial Group, Inc. and ATBancorp:

We are pleased to report that the boards of directors of MidWestOne Financial Group, Inc., referred to as MidWestOne, and ATBancorp, together referred to as the parties, have approved a strategic merger involving our two companies (the "Merger"). We cannot complete the combination without your approval. If the Merger proposals described in this document are approved by the shareholders of each party and the Merger is subsequently completed, ATBancorp will merge with and into MidWestOne, with MidWestOne as the surviving entity.

Pursuant to the terms of an Agreement and Plan of Merger dated August 21, 2018 (the "Merger Agreement"), each share of ATBancorp common stock issued and outstanding immediately prior to the effective time of the Merger (the "Effective Time"), excluding any Cancelled Shares and any Dissenters' Shares (each as defined in the Merger Agreement), shall represent the right to receive (i) 117.5500 fully paid and nonassessable shares of MidWestOne common stock (the "Per Share Stock Consideration") and (ii) $992.51 (the "Per Share Cash Consideration").

MidWestOne expects to issue approximately 4,117,541 shares of its common stock upon completion of the Merger. MidWestOne's common stock is traded on the NASDAQ Global Select Market under the symbol "MOFG." As reported on the NASDAQ Global Select Market, MidWestOne's common stock closed at $33.25 on August 21, 2018, the day before the announcement of the Merger, and at $28.98 on November 26, 2018, the last practicable trading day before the date of this joint proxy statement/prospectus. Based on these closing prices, the transaction value is estimated at $171.67 million, or $4,901.05 per share of ATBancorp common stock, or $154.09 million, or $4,399.11 per share of ATBancorp common stock, respectively. Based on a Final Acquiror Market Value (as defined below) of MidWestOne's common stock of $26.60 (the price beneath which ATBancorp would have the right to terminate the Merger Agreement, as described below, subject to the other conditions therein, including MidWestOne's right to increase the amount of consideration payable to ATBancorp shareholders), the transaction value is estimated at $144.29 million, or $4,119.34 per share of ATBancorp common stock. The ultimate value of the consideration received by ATBancorp shareholders will depend on the trading value of MidWestOne common stock prior to the closing of the Merger.

ATBancorp has the right to terminate the Merger Agreement upon a significant decrease in the price of MidWestOne's common stock. In particular, ATBancorp may terminate the Merger Agreement if, as of the first date on which all requisite regulatory approvals (and waivers, if applicable) necessary for consummation of the Merger have been received (disregarding any waiting period) (the "Determination Date"): (i) the weighted average of the daily closing sales prices of a share of MidWestOne's common stock as reported on the Nasdaq Global Select Market for the 20 consecutive trading days immediately preceding the Determination Date (the "Final Acquiror Market Value") is less than $26.60; and (ii) the number obtained by dividing the Final Acquiror Market Value by $33.25 is less than the quotient of (x) the average of the daily closing value of the NASDAQ Bank Index for the 20 consecutive trading days immediately preceding the Determination Date, and (y) $4,315.63 (such quotient, the "Index Ratio"), multiplied by 0.80. If ATBancorp provides notice of such termination to MidWestOne, MidWestOne may elect to increase the Per Share Stock Consideration to equal the lesser of (1) a quotient, the numerator of which is equal to the product of (A) $33.25; (B) the Per Share Stock Consideration; and (C) the Index Ratio multiplied by 0.80, and the denominator of which is equal to the Final Acquiror Market Value; or (2) the quotient determined by dividing the $33.25 by the Final Acquiror Market Value, and multiplying the quotient by the product of the Per Share Stock Consideration and 0.80.

The effect of ATBancorp's termination right and MidWestOne's right to increase consideration is to ensure that the total consideration paid to ATBancorp shareholders is at least $144.29 million.

The Merger is intended to qualify as a reorganization within the meaning of Section 368(a) of the Internal Revenue Code of 1986, as amended, (the "Internal Revenue Code"). Assuming the Merger qualifies as a reorganization, a shareholder of ATBancorp generally will not recognize any gain or loss upon receipt of MidWestOne common stock in exchange for ATBancorp common stock in the Merger, and will recognize gain (but not loss) in an amount not to exceed any cash received as part of the Merger consideration (except with respect to any cash received in lieu of a fractional share of MidWestOne common stock, as discussed below under "Material U.S. Federal Income Tax Consequences of the Merger—Cash Received In Lieu of a Fractional Share of MidWestOne Common Stock").

MidWestOne and ATBancorp will each hold a special meeting of shareholders to consider certain matters relating to the proposed Merger. MidWestOne and ATBancorp cannot complete the proposed Merger unless: (1) MidWestOne's shareholders vote to approve and adopt the Merger Agreement and the transactions contemplated thereby and approve the issuance of MidWestOne common stock in connection with the Merger, and (2) ATBancorp's shareholders vote to approve and adopt the Merger Agreement and the transactions contemplated thereby. The parties' respective boards of directors are providing this document to solicit your proxy to vote for approval of the parties' respective proposals.

This document is also being delivered to ATBancorp shareholders as MidWestOne's prospectus for its offering of MidWestOne common stock in connection with the Merger.

Your vote is very important. To ensure your representation at the MidWestOne or ATBancorp special meeting, as applicable, please complete and return the enclosed proxy card or submit your proxy by one of the other means described below. Whether or not you expect to attend the MidWestOne or ATBancorp special meeting, as applicable, please vote promptly. Submitting a proxy now will not prevent you from being able to vote in person at the applicable special meeting. Each of the MidWestOne and ATBancorp boards of directors has approved the Merger Agreement and the transactions contemplated thereby and recommends to its shareholders to vote "FOR" adoption or approval of their respective proposals.

This document provides you with detailed information about the proposed Merger. It also contains or references information about MidWestOne and ATBancorp and certain related matters. You are encouraged to read this document carefully. In particular, you should read the "Risk Factors" section beginning on page 44 for a discussion of the risks you should consider in evaluating the proposed Merger and how it will affect you.



Sincerely,

Charles N. Funk President and Chief Executive Officer of MidWestOne Financial Group, Inc.		Nicholas J. Schrup, III Chairman and President of ATBancorp

Neither the Securities and Exchange Commission nor any state securities commission has approved or disapproved of the Merger, the issuance of MidWestOne common stock in connection with the Merger or the other transactions described in this document, or passed upon the adequacy or accuracy of the disclosure in this document. Any representation to the contrary is a criminal offense.

The securities to be issued in connection with the Merger are not savings accounts, deposits or other obligations of any bank, non-bank subsidiary or savings association and are not insured by the Federal Deposit Insurance Corporation or any other governmental agency. The securities are subject to investment risk, including possible loss of principal.

This document is dated November 30, 2018 and is first being mailed to shareholders of MidWestOne and ATBancorp on or about December 5, 2018.

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WHERE YOU CAN FIND MORE INFORMATION

MidWestOne files annual, quarterly and special reports, proxy statements and other business and financial information with the Securities and Exchange Commission (the "SEC"). You may read and copy any materials that MidWestOne files with the SEC at the SEC's Public Reference Room at 100 F Street, N.E., Washington, D.C. 20549, at prescribed rates. Please call the SEC at (800) SEC-0330 for further information on the public reference room. In addition, MidWestOne files reports and other business and financial information with the SEC electronically, and the SEC maintains a website located at http://www.sec.gov containing this information. You will also be able to obtain these documents, free of charge, from MidWestOne at www.midwestone.com under the "About MidWestOne Financial Group," and the "SEC Filings" links.

MidWestOne has filed a registration statement on Form S-4 of which this document forms a part. As permitted by SEC rules, this document does not contain all of the information included in the registration statement or in the exhibits or schedules to the registration statement. You may read and copy the registration statement, including any amendments, schedules and exhibits, at the addresses set forth below. Statements contained in this document as to the contents of any contract or other documents referred to in this document are not necessarily complete. In each case, you should refer to the copy of the applicable contract or other document filed as an exhibit to the registration statement. This document incorporates by reference documents that MidWestOne has previously filed with the SEC. They contain important information about the company and its financial condition. For further information, please see the section entitled "Incorporation of Certain Documents by Reference" beginning on page 189. These documents are available without charge at its principal executive office, at the following address and telephone number:

MidWestOne Financial Group, Inc.
102 South Clinton Street
Iowa City, Iowa 52240
Attn: Kenneth R. Urmie, Corporate Secretary
(319) 356-5800

To obtain timely delivery of these documents, you must request the information no later than five business days before the date of the MidWestOne special meeting of shareholders in order to receive them before MidWestOne's special meeting and no later than five business days before the date of the ATBancorp special meeting of shareholders in order to receive them before ATBancorp's special meeting.

In addition, if you have questions about the Merger or the ATBancorp special meeting, need additional copies of this joint proxy statement/prospectus or need to obtain proxy cards or other information related to the proxy solicitation, you may contact ATBancorp at its principal executive office, at the following address and telephone number:

ATBancorp
895 Main Street
Dubuque, Iowa 52001
Attention: John W. Marshall, Secretary
(563) 589-7178

ATBancorp does not have a class of securities registered under Section 12 of the Securities Exchange Act of 1934, as amended (the "Exchange Act"), is not subject to the reporting requirements of Section 13(a) or 15(d) of the Exchange Act, and accordingly does not file documents or reports with the SEC.

You should rely only on the information contained or incorporated by reference in this joint proxy statement/prospectus. No one has been authorized to provide you with information that is different from what is contained in this joint proxy statement/prospectus. You should not assume that the information contained in this joint proxy statement/prospectus is accurate as of any date other than the date of this joint proxy statement/prospectus, and neither the mailing of this joint proxy

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statement/prospectus to MidWestOne and ATBancorp shareholders nor the issuance of MidWestOne common stock in the Merger shall create any implication to the contrary.

Information on the websites of MidWestOne or ATBancorp, or any subsidiary of MidWestOne or ATBancorp, is not part of this document or incorporated by reference herein. You should not rely on that information in deciding how to vote.

This document does not constitute an offer to sell, or a solicitation of an offer to buy, any securities, or the solicitation of a proxy, in any jurisdiction to or from any person to whom it is unlawful to make any such offer or solicitation in such jurisdiction. Except where the context otherwise indicates, information contained in this document regarding MidWestOne has been provided by MidWestOne and information contained in this document regarding ATBancorp has been provided by ATBancorp.

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102 South Clinton St.
Iowa City, Iowa 52240
(319) 356-5800

NOTICE OF THE SPECIAL MEETING OF SHAREHOLDERS
TO BE HELD ON JANUARY 11, 2019

NOTICE IS HEREBY GIVEN that a special meeting of the shareholders of MidWestOne Financial Group, Inc. ("MidWestOne"), will be held at 102 South Clinton Street, Summerwill Conference Room, Iowa City, IA 52240, at 10:00 a.m., Central Time, on Friday, January 11, 2019 for the following purposes:

1.: Approval and adoption of the Agreement and Plan of Merger, dated as of August 21, 2018, by and between MidWestOne Financial Group, Inc., or MidWestOne, and ATBancorp, as such agreement may be amended from time to time (the "Merger Agreement"), and the transactions contemplated thereby (the "MidWestOne merger proposal");
2.: Approval of the issuance of MidWestOne common stock in the Merger (the "MidWestOne stock issuance proposal"); and
3.: Approval of one or more adjournments of the MidWestOne special meeting, if necessary or appropriate, including adjournments to permit further solicitation of proxies in favor of the MidWestOne merger proposal and the MidWestOne stock issuance proposal (the "MidWestOne adjournment proposal").

MidWestOne will transact no other business at the special meeting, except for business properly brought before the special meeting or any adjournment or postponement thereof.

The above proposals are described in more detail in this document, which you should read carefully in its entirety before you vote. A copy of the Merger Agreement is attached as Appendix A to this document.

The MidWestOne board of directors has set November 26, 2018 as the record date for the MidWestOne special meeting. Only holders of record of MidWestOne common stock at the close of business on November 26, 2018 will be entitled to notice of and to vote at the MidWestOne special meeting and any adjournments or postponements thereof. Any shareholder entitled to attend and vote at the MidWestOne special meeting is entitled to appoint a proxy to attend and vote on such shareholder's behalf. Such proxy need not be a holder of MidWestOne common stock.

Your vote is very important. To ensure your representation at the MidWestOne special meeting, please complete and return the enclosed proxy card or submit your proxy by telephone, through the Internet or by mail. Please vote promptly whether or not you expect to attend the MidWestOne special meeting. Submitting a proxy now will not prevent you from being able to vote in person at the MidWestOne special meeting.

The MidWestOne board of directors has approved the Merger Agreement and the transactions contemplated thereby and recommends that you vote "FOR" the MidWestOne merger proposal, "FOR" the MidWestOne stock issuance proposal and "FOR" the MidWestOne adjournment proposal (if necessary or appropriate).

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BY ORDER OF THE BOARD OF DIRECTORS

Charles N. Funk
President and Chief Executive Officer of
MidWestOne Financial Group, Inc.

Iowa City, Iowa
November 30, 2018

PLEASE VOTE YOUR SHARES OF MIDWESTONE COMMON STOCK PROMPTLY. YOU CAN FIND INSTRUCTIONS FOR VOTING ON THE ENCLOSED PROXY CARD. IF YOU HAVE QUESTIONS ABOUT THE PROPOSALS OR ABOUT VOTING YOUR SHARES, PLEASE CALL MIDWESTONE INVESTOR RELATIONS AT (319) 356-5800.

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ATBANCORP
895 MAIN STREET
DUBUQUE, IOWA 52001
(563) 582-1841

NOTICE OF THE SPECIAL MEETING OF SHAREHOLDERS
TO BE HELD ON JANUARY 10, 2019

To the holders of ATBancorp common stock:

NOTICE IS HEREBY GIVEN that a special meeting of the shareholders of ATBancorp will be held at American Trust & Savings Bank, 895 Main Street, Dubuque, Iowa 52001, at 11:00 a.m., Central Time, on Thursday, January 10, 2019 for the following purposes:

1.: Approval and adoption of the Agreement and Plan of Merger, dated as of August 21, 2018, by and between MidWestOne Financial Group, Inc. and ATBancorp, as such agreement may be amended from time to time (the "Merger Agreement"), and the transactions contemplated thereby (the "ATBancorp merger proposal"); and
2.: Approval of one or more adjournments of the ATBancorp special meeting, if necessary or appropriate, including adjournments to permit further solicitation of proxies in favor of the ATBancorp merger proposal (the "ATBancorp adjournment proposal").

ATBancorp will transact no other business at the special meeting, except for business properly brought before the special meeting or any adjournment or postponement thereof.

The ATBancorp board of directors has set November 15, 2018 as the record date for the ATBancorp special meeting. Only holders of record of ATBancorp common stock at the close of business on November 15, 2018 will be entitled to notice of and to vote at the ATBancorp special meeting and any adjournments or postponements thereof. Any shareholder entitled to attend and vote at the ATBancorp special meeting is entitled to appoint a proxy to attend and vote on such shareholder's behalf. Such proxy need not be a holder of ATBancorp common stock.

Your vote is very important. To ensure your representation at the ATBancorp special meeting, please complete and return the enclosed proxy card. Please vote promptly whether or not you expect to attend the ATBancorp special meeting. Submitting a proxy now will not prevent you from being able to vote in person at the ATBancorp special meeting.

The ATBancorp board of directors has approved the Merger Agreement and the transactions contemplated thereby and recommends that you vote "FOR" the ATBancorp merger proposal and "FOR" the ATBancorp adjournment proposal (if necessary or appropriate).

BY ORDER OF THE BOARD OF DIRECTORS

Nicholas J. Schrup, III
Chairman and President of ATBancorp

Dubuque, Iowa
November 30, 2018

PLEASE VOTE YOUR SHARES OF ATBANCORP COMMON STOCK PROMPTLY. YOU CAN FIND INSTRUCTIONS FOR VOTING ON THE ENCLOSED PROXY CARD. IF YOU HAVE QUESTIONS ABOUT THE PROPOSALS OR ABOUT VOTING YOUR SHARES, PLEASE CALL JOHN W. MARSHALL, SECRETARY, AT (563) 589-7178.

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QUESTIONS AND ANSWERS ABOUT THE MERGER AND THE SPECIAL MEETINGS			1
SUMMARY			11
CERTAIN FINANCIAL INFORMATION REGARDING MIDWESTONE AND ATBANCORP			22
RISK FACTORS			44
CAUTIONARY STATEMENT REGARDING FORWARD-LOOKING STATEMENTS			51
MIDWESTONE SPECIAL MEETING OF SHAREHOLDERS			53
MIDWESTONE PROPOSALS			57
ATBANCORP SPECIAL MEETING OF SHAREHOLDERS			59
ATBANCORP PROPOSALS			62
THE MERGER			63
THE MERGER AGREEMENT			113
MATERIAL U.S. FEDERAL INCOME TAX CONSEQUENCES OF THE MERGER			129
COMPARISON OF SHAREHOLDERS' RIGHTS			134
INFORMATION ABOUT MIDWESTONE			141
INFORMATION ABOUT ATBANCORP			142
MANAGEMENT'S DISCUSSION AND ANALYSIS OF FINANCIAL CONDITION AND RESULTS OF OPERATIONS			155
STOCK OWNERSHIP OF DIRECTORS AND EXECUTIVE OFFICERS			180
DESCRIPTION OF MIDWESTONE CAPITAL STOCK			182
EXPERTS			186
OPINIONS			187
OTHER MATTERS			188
INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE			189
ATBANCORP FINANCIAL STATEMENTS			F-1


Appendix A		Merger Agreement
Appendix B		Iowa Appraisal Rights Statute
Appendix C		Opinion of MidWestOne's Financial Advisor
Appendix D		Opinion of ATBancorp's Financial Advisor
Appendix E		Form of Voting Agreement
Appendix F		Form of Lock-Up Agreement

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QUESTIONS AND ANSWERS ABOUT THE MERGER AND THE SPECIAL MEETINGS

The following are answers to certain questions that you may have regarding the special meetings. The parties urge you to read carefully the remainder of this document because the information in this section may not provide all the information that might be important to you in determining how to vote. Additional important information is also contained in the appendices to, and the documents incorporated by reference in, this document.

Q:: WHAT IS THE MERGER?
A.: MidWestOne and ATBancorp have entered into a Merger Agreement pursuant to which, subject to the terms and conditions of the Merger Agreement, ATBancorp will merge with and into MidWestOne, with MidWestOne continuing as the surviving corporation (the "Merger"). A copy of the Merger Agreement is attached as Appendix A to this document. Immediately following the Merger, each of American Trust & Savings Bank ("ATSB"), an Iowa state chartered bank, and American Bank & Trust Wisconsin ("ABTW"), a Wisconsin state chartered bank, each of which is a wholly-owned subsidiary of ATBancorp, will merge with and into MidWestOne Bank, an Iowa state chartered bank and a wholly owned subsidiary of MidWestOne ("MidWestOne Bank"), with MidWestOne Bank continuing as the surviving bank (the "Bank Mergers"). In order to complete the transaction, each party needs not only the approval of its respective shareholders but the approval of each of these mergers by the applicable banking regulators of MidWestOne, MidWestOne Bank, ATBancorp, ATSB and ABTW.
Q:: WHY AM I RECEIVING THIS JOINT PROXY STATEMENT/PROSPECTUS?
A.: Each of MidWestOne and ATBancorp is sending these materials to its shareholders to help them decide how to vote their shares of MidWestOne or ATBancorp common stock, as the case may be, with respect to the matters to be considered at the special meetings.

The Merger cannot be completed unless MidWestOne shareholders approve and adopt the Merger Agreement and the transactions contemplated thereby and approve the issuance of MidWestOne common stock in the Merger, and ATBancorp shareholders approve and adopt the Merger Agreement and the transactions contemplated thereby. Each of MidWestOne and ATBancorp is holding a special meeting of its shareholders to vote on the proposals necessary to complete the Merger. Information about these special meetings, the Merger and the business to be considered by shareholders at each of the special meetings is contained in this document.

This document constitutes both a joint proxy statement of MidWestOne and ATBancorp and a prospectus of MidWestOne. It is a joint proxy statement because each of the boards of directors of MidWestOne and ATBancorp is soliciting proxies using this document from their respective shareholders. It is a prospectus because MidWestOne, in connection with the Merger, is offering shares of its common stock in exchange for outstanding shares of ATBancorp common stock.

Q:: WHAT WILL ATBANCORP SHAREHOLDERS RECEIVE IN THE MERGER?
A:: Each share of ATBancorp common stock issued and outstanding immediately prior to the effective time of the Merger ("Effective Time"), excluding any Cancelled Shares and any Dissenters' Shares (each as defined in the Merger Agreement), shall represent the right to receive (i) the Per Share Stock Consideration and (ii) the Per Share Cash Consideration.

For each fractional share that would otherwise be issued, MidWestOne will pay cash in an amount equal to such fraction multiplied by the volume weighted average of the daily closing sales prices of a share of MidWestOne's common stock as reported on the Nasdaq Global Select Market for the 20 consecutive trading days immediately preceding the Closing Date (the "Closing Acquiror

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Common Stock Price"). The "Closing Date" will be the first day of the month immediately following the month in which the satisfaction or waiver of the latest to occur of MidWestOne's and ATBancorp's respective closing conditions occurs, or at such other time and place as MidWestOne and ATBancorp may agree. No interest will be paid or accrue on cash payable to holders in lieu of fractional shares.

Based on MidWestOne's common stock's closing price of $33.25 on August 21, 2018, the day before the announcement of the Merger, the transaction value is estimated at $171.67 million, or $4,901.05 per share of ATBancorp common stock. Based on MidWestOne's common stock's closing price of $28.98 on November 26, 2018, the transaction value is estimated at $154.09 million, or $4,399.11 per share of ATBancorp common stock. Based on a Final Acquiror Market Value of MidWestOne's closing stock of $26.60 (the price beneath which ATBancorp would have the right to terminate the Merger Agreement, subject to the other conditions therein, including MidWestOne's right to increase the amount of consideration payable to ATBancorp shareholders), the transaction value is estimated at $144.29 million, or $4,119.34 per share of ATBancorp common stock.

The ultimate value of the consideration received by ATBancorp shareholders will depend on the trading value of MidWestOne common stock prior to the closing of the Merger.

Q:: WILL THE VALUE OF THE MERGER CONSIDERATION CHANGE BETWEEN THE DATE OF THIS DOCUMENT AND THE TIME THE MERGER IS COMPLETED?
A:: Yes. The trading price of MidWestOne common stock may increase or decrease prior to the Closing Date, and the value of the shares of MidWestOne common stock that you would receive in connection with the Merger would increase or decrease accordingly. In addition, the number of shares of MidWestOne common stock that you may receive in the Merger may be increased in certain circumstances pursuant to the terms of the Merger Agreement. See "The Merger Agreement—Termination of the Merger Agreement."
Q:: WILL ATBANCORP SHAREHOLDERS BE ABLE TO TRADE THE SHARES OF MIDWESTONE COMMON STOCK RECEIVED IN THE MERGER?
A:: Yes. The MidWestOne common stock issued in the Merger to ATBancorp shareholders, except for holders of any Dissenters' Shares, will be registered under the Securities Act of 1933, as amended (the "Securities Act"), and will be listed on NASDAQ under the symbol "MOFG." All shares of MidWestOne common stock issued in the Merger will be freely transferable and will not be subject to any restrictions on transfer arising under the Securities Act, except for (i) shares issued to any ATBancorp shareholder who may be deemed to be an "affiliate" of MidWestOne after completion of the Merger, or (ii) shares subject to a Lock-Up Agreement described below. An affiliate of a corporation, as defined by the rules promulgated under the Securities Act, is a person who directly or indirectly, through one or more intermediaries, controls, is controlled by, or is under common control with, that corporation. Affiliates generally include directors, executive officers and beneficial owners of 10% or more of a corporation's capital stock.

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disposition (whether by actual disposition or effective economic disposition or otherwise) of any shares of MidWestOne common stock whether owned as of the time of the Lock-Up Agreements or subsequently acquired, including shares of MidWestOne common stock received by the shareholder pursuant to the Merger Agreement; (ii) enter into any swap or other derivative transaction that transfers to another, in whole or in part, any of the economic benefits or risks of ownership of MidWestOne common stock, whether any such transaction is to be settled by delivery of MidWestOne common stock or other securities, in cash or otherwise; or (iii) publicly disclose an intention to effect any transaction contemplated by clause (i) or (ii). If the Merger Agreement is terminated without the consummation of the Merger, the Lock-Up Agreements will automatically terminate.

Former ATBancorp shareholders who are not affiliates of MidWestOne after the completion of the Merger and are not subject to the terms of a Lock-Up Agreement may sell their shares of MidWestOne common stock received in the Merger at any time. Former ATBancorp shareholders who become affiliates of MidWestOne after completion of the Merger and are not subject to the terms of a Lock-Up Agreement will be subject to the volume and sale limitations of Rule 144 under the Securities Act until they are no longer affiliates of MidWestOne. This joint proxy statement/prospectus does not cover resales of MidWestOne common stock received by any person upon completion of the Merger, and no person is authorized to make any use of or rely on this joint proxy statement/prospectus in connection with or to effect any resale of MidWestOne common stock.

Q:: WHAT ARE THE SPECIAL DIVIDENDS?
A:: The Merger Agreement provides that ATBancorp may pay a special dividend of $907.8452 per share to the ATBancorp shareholders at any time prior to the closing of the Merger (the "Closing") as determined by ATBancorp. Such dividend represents a portion of the proceeds to ATBancorp from the sale of Heritage Commerce Corp common stock received as consideration in connection with the sale of United American Bank. The special dividend was distributed to ATBancorp shareholders on September 24, 2018.

The Merger Agreement also provides that ATBancorp may pay a special dividend to shareholders based on the net proceeds (if any) received by ATBancorp in connection with the disposition by ATBancorp of certain of its businesses—American Trust Retirement, ATBancorp's California trust office, and ATInsurance—as well as 100% of the equity of ATCapital Management, a registered investment advisor and a wholly owned subsidiary of ATBancorp (collectively, the "ATB Businesses"), prior to Closing, as required by the Merger Agreement. The payment of these special dividends by ATBancorp is in addition to the Merger consideration to be paid by MidWestOne. ATBancorp shareholders are not required to take any action to approve or receive these special dividends.

As required by the Merger Agreement, ATBancorp is currently in the process of negotiating with third parties the sale of the ATB Businesses. One purchaser would purchase the assets derived out of ATBancorp's California trust office and the other purchaser would purchase the rest of the assets, which make up a bulk of the ATB Businesses. As of the date of this joint proxy statement/prospectus, ATBancorp has not entered into a definitive agreement with the potential purchasers. As a result, it is not known when any such sale may close or whether ATBancorp will ultimately sell the ATB Businesses to the potential purchasers with whom it is in negotiations. It is also likely that any such sales will require the establishment of escrow accounts with the proceeds from the sales of the ATB Businesses. The establishment of any escrows will likely delay the time of a full distribution of the net proceeds from the sale of the ATB Businesses and any claims against the escrow accounts could materially decrease the amount of consideration payable to ATBancorp shareholders that they would have otherwise been entitled to through a special dividend.

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Q:: WHEN WILL THE MERGER BE COMPLETED?
A:: MidWestOne and ATBancorp are working to complete the Merger as soon as practicable. The Merger will occur on the first day of the month immediately following the month in which the satisfaction or waiver of the latest to occur of the closing conditions set forth in the Merger Agreement or at such other time and place as MidWestOne and ATBancorp may agree. Neither MidWestOne nor ATBancorp can predict the actual date on which the Merger will be completed because it is subject to factors beyond each company's control, including whether or when the parties' respective shareholders' approvals will be received. For further information, please see the section entitled "The Merger Agreement—Conditions to Consummation of the Merger" beginning on page 124.
Q:: WHO IS ENTITLED TO VOTE?
A:: MidWestOne Special Meeting. Holders of record of MidWestOne common stock at the close of business on November 26, 2018, which is the date that the MidWestOne board of directors has fixed as the record date for the MidWestOne special meeting, are entitled to vote at the MidWestOne special meeting.

ATBancorp Special Meeting. Holders of record of ATBancorp common stock at the close of business on November 15, 2018, which is the date that the ATBancorp board of directors has fixed as the record date for the ATBancorp special meeting, are entitled to vote at the ATBancorp special meeting.

Q:: WHAT CONSTITUTES A QUORUM?
A:: MidWestOne Special Meeting. A majority of the outstanding shares of MidWestOne entitled to vote, represented in person or by proxy, will constitute a quorum for the transaction of business at the MidWestOne special meeting. Abstentions and broker non-votes, if any, will be included in determining the number of shares present at the meeting for the purpose of determining the presence of a quorum.

ATBancorp Special Meeting. A majority of the outstanding shares of ATBancorp entitled to vote, represented in person or by proxy, will constitute a quorum for the transaction of business at the ATBancorp special meeting. Abstentions will be included in determining the number of shares present at the meeting for the purpose of determining the presence of a quorum.

Q:

WHAT AM I BEING ASKED TO VOTE ON AND WHY IS THIS APPROVAL NECESSARY?

A:

MidWestOne Special Meeting. MidWestOne shareholders are being asked to vote on the following proposals:

1.: To approve the MidWestOne merger proposal.
2.: To approve the MidWestOne stock issuance proposal.
3.: To approve the MidWestOne adjournment proposal (if necessary or appropriate).

Shareholder approval of the MidWestOne merger proposal and the MidWestOne stock issuance proposal is required to complete the Merger. MidWestOne will transact no business other than as listed above at the MidWestOne special meeting, except for business properly brought before the MidWestOne special meeting or any adjournment or postponement thereof.

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ATBancorp Special Meeting. ATBancorp shareholders are being asked to vote on the following proposals:

1.: To approve the ATBancorp merger proposal.
2.: To approve the ATBancorp adjournment proposal (if necessary or appropriate).

Shareholder approval of the ATBancorp merger proposal is required to complete the Merger. ATBancorp will transact no business other than as listed above at the ATBancorp special meeting, except for business properly brought before the ATBancorp special meeting or any adjournment or postponement thereof.

Q:: WHAT VOTE IS REQUIRED TO APPROVE EACH PROPOSAL AT THE MIDWESTONE SPECIAL MEETING?
A:: The MidWestOne merger proposal: The affirmative vote of a majority of the votes entitled to be cast on the proposal is required to approve the MidWestOne merger proposal.

The MidWestOne stock issuance proposal: The affirmative vote of a majority of the shares of MidWestOne common stock represented in person or by proxy at the MidWestOne special meeting and entitled to vote on the proposal is required to approve the MidWestOne stock issuance proposal.

The MidWestOne adjournment proposal: The affirmative vote of a majority of the shares of MidWestOne common stock represented in person or by proxy at the MidWestOne special meeting and entitled to vote on the proposal is required to approve the MidWestOne adjournment proposal.

Q:: WHAT DOES THE MIDWESTONE BOARD OF DIRECTORS RECOMMEND?
A:: The MidWestOne board of directors recommends that MidWestOne shareholders vote "FOR" the MidWestOne merger proposal, "FOR" the MidWestOne stock issuance proposal and "FOR" the MidWestOne adjournment proposal (if necessary or appropriate).
Q:: WHAT VOTE IS REQUIRED TO APPROVE EACH PROPOSAL AT THE ATBANCORP SPECIAL MEETING?
A:: The ATBancorp merger proposal: The affirmative vote of a majority of the shares of ATBancorp common stock represented in person or by proxy at the ATBancorp special meeting and entitled to vote on the proposal is required to approve the ATBancorp merger proposal. Pursuant to the Voting Agreement (as defined below), approval of the ATBancorp merger proposal is virtually assured.

The ATBancorp adjournment proposal: The affirmative vote of a majority of the shares of ATBancorp common stock represented in person or by proxy at the ATBancorp special meeting and entitled to vote on the proposal is required to approve the ATBancorp adjournment proposal.

Q:: WHAT DOES THE ATBANCORP BOARD OF DIRECTORS RECOMMEND?
A:: The ATBancorp board of directors recommends that ATBancorp shareholders vote "FOR" the ATBancorp merger proposal and "FOR" the ATBancorp adjournment proposal (if necessary or appropriate).

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Q:

WHAT DO I NEED TO DO NOW?

A:

After carefully reading and considering the information contained in this document, please vote your shares as soon as possible so that your shares will be represented at your respective company's special meeting. Please follow the instructions set forth on the proxy card or on the voting instruction form provided by the record holder if your shares are held in the name of your broker, bank or other nominee.

Q:

HOW DO I VOTE?

A:

MidWestOne Special Meeting. If you are a shareholder of record of MidWestOne as of the MidWestOne record date, you may submit your proxy before MidWestOne's special meeting in one of the following ways:

•: use the toll-free number shown on your proxy card;
•: visit the website shown on your proxy card to vote via the Internet; or
•: complete, sign, date and return the enclosed proxy card in the enclosed postage-paid envelope.

You may also cast your vote in person at the MidWestOne special meeting.

If your shares are held in "street name," through a broker, bank or other nominee, that institution will send you separate instructions describing the procedure for voting your shares. "Street name" shareholders who wish to vote at the meeting will need to obtain a proxy form from their broker, bank or other nominee.

ATBancorp Special Meeting. If you are a shareholder of ATBancorp as of the ATBancorp record date, you may submit your proxy before ATBancorp's special meeting by completing, signing, dating and returning the enclosed proxy card in the enclosed postage-paid envelope.

You may also cast your vote in person at the ATBancorp special meeting.

Q:: HOW MANY VOTES DO I HAVE?
A:: MidWestOne Shareholders. You are entitled to one vote for each share of MidWestOne common stock that you owned as of the MidWestOne record date. As of the close of business on the MidWestOne record date, there were approximately 12,222,145 outstanding shares of MidWestOne common stock entitled to vote. As of that date, approximately 3.2% of such outstanding shares of MidWestOne common stock were beneficially owned by the directors and executive officers of MidWestOne and their respective affiliates.

ATBancorp Shareholders. You are entitled to one vote for each share of ATBancorp common stock that you owned as of the ATBancorp record date. As of the close of business on the ATBancorp record date, there were 35,028 outstanding shares of ATBancorp common stock entitled to vote. As of that date, approximately 66.6% of such outstanding shares of ATBancorp common stock were beneficially owned by the directors and executive officers of ATBancorp and their respective affiliates. In addition, shareholders owning 62.41% of the outstanding shares of ATBancorp common stock are parties to a voting and support agreement with MidWestOne (the "Voting Agreement") requiring them to vote in favor of the ATBancorp merger proposal.

Q:: WHEN AND WHERE ARE THE MIDWESTONE AND ATBANCORP SPECIAL MEETINGS?
A:: The special meeting of MidWestOne shareholders will be held at 102 South Clinton Street, Summerwill Conference Room, Iowa City, IA 52240 at 10:00 a.m., Central Time, on Friday, January 11, 2019. Subject to space availability, all MidWestOne shareholders as of the MidWestOne record date, or their duly appointed proxies, may attend the MidWestOne special meeting.

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The special meeting of ATBancorp shareholders will be held at American Trust & Savings Bank, 895 Main Street, Dubuque, Iowa 52001 at 11:00 a.m., Central Time, on Thursday, January 10, 2019. All ATBancorp shareholders as of the ATBancorp record date, or their duly appointed proxies, may attend the ATBancorp special meeting.

Q:: IF MY MIDWESTONE SHARES ARE HELD IN "STREET NAME" BY A BROKER, BANK OR OTHER NOMINEE, WILL MY BROKER, BANK OR OTHER NOMINEE VOTE MY SHARES FOR ME?
A:: If you are a MidWestOne shareholder and your shares are held in "street name" in a stock brokerage account or by a bank or other nominee, you must provide the record holder of your shares with instructions on how to vote your shares. Please follow the voting instructions provided by your broker, bank or other nominee. Please note that you may not vote shares held in street name by returning a proxy card directly to MidWestOne or by voting in person at the MidWestOne special meeting unless you provide a "legal proxy," which you must obtain from your broker, bank or other nominee.

Under the rules of NASDAQ, brokers who hold shares in street name for a beneficial owner of those shares typically have the authority to vote in their discretion on "routine" proposals when they have not received instructions from beneficial owners. However, brokers are not allowed to exercise their voting discretion with respect to the approval of matters that NASDAQ determines to be "non-routine" without specific instructions from the beneficial owner. It is expected that all proposals to be voted on at the MidWestOne special meeting will be "non-routine" matters. Broker non-votes occur when a broker or nominee is not instructed by the beneficial owner of shares to vote on a particular proposal for which the broker does not have discretionary voting power.

If you are a MidWestOne shareholder and you hold your shares in street name and you do not instruct your broker, bank or other nominee on how to vote your shares:

•: your broker, bank or other nominee may not vote your shares on the MidWestOne merger proposal, which broker non-votes will have no effect on the vote count for such proposal;
•: your broker, bank or other nominee may not vote your shares on the MidWestOne stock issuance proposal, which broker non-votes will have no effect on the vote count for such proposal; and
•: your broker, bank or other nominee may not vote your shares on the MidWestOne adjournment proposal, which broker non-votes will have no effect on the vote count for such proposal.

Q:: HOW WILL MY MIDWESTONE SHARES HELD IN THE EMPLOYEE STOCK OWNERSHIP PLAN BE VOTED?
A:: MidWestOne maintains an employee stock ownership plan ("ESOP") that as of the MidWestOne record date owns 286,444 or 2.34% of the outstanding shares of MidWestOne's common stock. Employees of MidWestOne and MidWestOne Bank participate in the ESOP. As of the MidWestOne record date, 286,444 shares have been allocated to ESOP participants. Each ESOP participant has the right to instruct the trustee of the plan how to vote the shares of MidWestOne common stock allocated to his or her account under the ESOP. If an ESOP participant properly executes the voting instruction card, the ESOP trustee will vote the participant's shares in accordance with the participant's instructions. Shares of MidWestOne's common stock held in the ESOP, but not allocated to any participant's account, and allocated shares for which no voting instructions are received from participants, will be voted by the trustee in proportion to the results of the votes cast on the issue by the participants and beneficiaries.

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Q:: WHAT IF I DO NOT VOTE OR I ABSTAIN?
A:: For purposes of each of the MidWestOne special meeting and the ATBancorp special meeting, an abstention occurs when a shareholder attends the applicable special meeting, either in person or represented by proxy, but abstains from voting.

For the MidWestOne merger proposal, if a MidWestOne shareholder present in person at the MidWestOne special meeting abstains from voting, or responds by proxy with an "abstain" vote, it will have the same effect as a vote cast "AGAINST" this proposal. If a MidWestOne shareholder is not present in person at the MidWestOne special meeting at which there is a quorum and does not respond by proxy, it will have no effect on the vote count for such proposal.

For the MidWestOne stock issuance proposal and the MidWestOne adjournment proposal, if a MidWestOne shareholder present in person at the MidWestOne special meeting abstains from voting, or responds by proxy with an "abstain" vote, it will have the same effect as a vote cast "AGAINST" each such proposal. If a MidWestOne shareholder is not present in person at the MidWestOne special meeting at which there is a quorum and does not respond by proxy, it will have no effect on the vote count for each such proposal.

For the ATBancorp merger proposal, if an ATBancorp shareholder present in person at the ATBancorp special meeting abstains from voting, or responds by proxy with an "abstain" vote, it will have the same effect as a vote cast "AGAINST" this proposal. If an ATBancorp shareholder is not present in person at the ATBancorp special meeting at which there is a quorum and does not respond by proxy, it will have no effect on the vote count for this proposal.

For the ATBancorp adjournment proposal, if an ATBancorp shareholder present in person at the ATBancorp special meeting abstains from voting, or responds by proxy with an "abstain" vote, it will have the same effect as a vote cast "AGAINST" this proposal. If an ATBancorp shareholder is not present in person at the ATBancorp special meeting at which there is a quorum and does not respond by proxy, it will have no effect on the vote count for this proposal.

Q:

WHAT WILL HAPPEN IF I RETURN MY PROXY OR VOTING INSTRUCTION CARD WITHOUT INDICATING HOW TO VOTE?

A:

If you sign and return your proxy or voting instruction card without indicating how to vote on any particular proposal, the MidWestOne common stock represented by your proxy will be voted as recommended by the MidWestOne board of directors with respect to each MidWestOne proposal and the ATBancorp common stock represented by your proxy will be voted as recommended by the ATBancorp board of directors with respect to each ATBancorp proposal. Unless a MidWestOne shareholder or an ATBancorp shareholder, as applicable, checks the box on its proxy card to withhold discretionary authority, the proxyholders may use their discretion to vote on other matters relating to the MidWestOne special meeting or ATBancorp special meeting, as applicable.

Q:

MAY I CHANGE MY VOTE AFTER I HAVE DELIVERED MY PROXY OR VOTING INSTRUCTION CARD?

A:

MidWestOne Shareholders. Yes. You may change your vote at any time before your proxy is voted at the MidWestOne special meeting. You may do this in one of four ways:

•: by sending a notice of revocation to the corporate secretary of MidWestOne;
•: by logging onto the Internet website specified on your proxy card in the same manner you would to submit your proxy electronically or by calling the telephone number specified on your proxy card, in each case if you are eligible to do so and following the instructions on the proxy card;
•: by sending a completed proxy card bearing a later date than your original proxy card; or

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•: by attending the MidWestOne special meeting and voting in person.

If you choose any of the first three methods, you must take the described action such that the notice, internet vote or proxy card, as applicable, is received no later than the beginning of the MidWestOne special meeting.

If your shares are held in an account at a broker, bank or other nominee, you should contact your broker, bank or other nominee to change your vote.

ATBancorp Shareholders. Yes. You may change your vote at any time before your proxy is voted at the ATBancorp special meeting. You may do this in one of three ways:

•: by sending a notice of revocation to the corporate secretary of ATBancorp;
•: by sending a completed proxy card bearing a later date than your original proxy card; or
•: by attending the ATBancorp special meeting and voting in person.

If you choose either of the first two methods, you must take the described action such that the notice or proxy card, as applicable, is received no later than the beginning of the ATBancorp special meeting.

Q:: DO I NEED IDENTIFICATION TO ATTEND THE MIDWESTONE MEETING IN PERSON?
A:: Yes. Please bring proper identification, together with proof that you are a record owner of MidWestOne. If your shares of MidWestOne common stock are held in street name, please bring acceptable proof of ownership, such as a letter from your broker or an account statement showing that you beneficially owned shares of MidWestOne common stock on the record date.
Q:: ARE ATBANCORP SHAREHOLDERS ENTITLED TO APPRAISAL RIGHTS?
A:: Yes. ATBancorp shareholders will have the right to assert appraisal of their shares of ATBancorp common stock and obtain payment in cash for the fair value of their shares, but only if they perfect their dissenters' rights and comply with the applicable provisions of Iowa law. A copy of the Iowa statutory provisions related to appraisal rights is attached as Appendix B to this document, and a summary of these provisions can be found under "The Merger—ATBancorp Shareholder Appraisal Rights" beginning on page 109. Due to the complexity of the procedures for exercising the right to seek appraisal, ATBancorp shareholders who are considering exercising such rights are encouraged to seek the advice of legal counsel. Failure to strictly comply with the applicable Iowa law provisions will result in the loss of the right of appraisal.
Q:: WHAT ARE THE MATERIAL U.S. FEDERAL INCOME TAX CONSEQUENCES OF THE MERGER TO ATBANCORP SHAREHOLDERS?
A:: The Merger is intended to qualify, and the obligation of each party to complete the Merger is conditioned upon the party's receipt of an opinion to the effect that the Merger will qualify as a reorganization within the meaning of Section 368(a) of the Internal Revenue Code.

Accordingly, ATBancorp shareholders are not expected to recognize any gain or loss upon receipt of MidWestOne common stock in exchange for ATBancorp common stock in the Merger, and will recognize gain (but not loss) in an amount not to exceed any cash received as part of the Merger consideration (except with respect to any cash received in lieu of a fractional share of MidWestOne common stock, as discussed below under "Material U.S. Federal Income Tax Consequences of the Merger—Cash Received In Lieu of a Fractional Share of MidWestOne Common Stock"). For a more detailed discussion of the material U.S. federal income tax consequences of the transaction,

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please see the section entitled "Material U.S. Federal Income Tax Consequences of the Merger" beginning on page 129.

The tax consequences of the Merger to any particular shareholder will depend on that shareholder's particular facts and circumstances. Accordingly, you are urged to consult your tax advisor to determine your tax consequences from the Merger.

Q:: WHAT HAPPENS IF THE MERGER IS NOT COMPLETED?
A:: If the Merger Agreement is terminated and the Merger is not completed, ATBancorp shareholders will not receive any consideration for their shares of ATBancorp common stock that otherwise would have been received in connection with the Merger. Instead, ATBancorp will remain an independent company. In addition, if the Merger Agreement is terminated under certain circumstances, ATBancorp would be required to pay MidWestOne a termination fee of $7.6 million, plus reimburse MidWestOne's expenses up to $1,000,000.
Q:: SHOULD ATBANCORP SHAREHOLDERS SEND IN THEIR STOCK CERTIFICATES NOW?
A:: No. ATBancorp shareholders should not send in any stock certificates now. If the Merger is approved, transmittal materials with instructions for their completion will be provided to ATBancorp shareholders under separate cover and the stock certificates should be sent at that time.
Q:: WHAT SHOULD I DO IF I RECEIVE MORE THAN ONE SET OF VOTING MATERIALS?
A:: ATBancorp and MidWestOne shareholders may receive more than one set of voting materials, including multiple copies of this document and multiple proxy cards or voting instruction cards. For example, if you hold shares of MidWestOne common stock in more than one brokerage account, you will receive a separate voting instruction card for each brokerage account in which you hold such shares. If you are a holder of record of ATBancorp common stock or MidWestOne common stock and your shares are registered in more than one name, you will receive one or more separate proxy cards or voting instruction cards for each company. Please complete, sign, date and return each proxy card and voting instruction card that you receive or otherwise follow the voting instructions set forth in this document to ensure that you vote every share of ATBancorp common stock or MidWestOne common stock that you own.
Q:: WHO SHOULD I CONTACT IF I HAVE ANY QUESTIONS ABOUT THE PROXY MATERIALS OR VOTING?
A:: If you are a MidWestOne shareholder and have any questions about the proxy materials or if you need assistance submitting your proxy or voting your shares or need additional copies of this document or the enclosed proxy card, you should contact MidWestOne Investor Relations at (319) 356-5800.

If you are an ATBancorp shareholder and have any questions about the proxy materials or if you need assistance submitting your proxy or voting your shares or need additional copies of this document or the enclosed proxy card, you should contact John W. Marshall, Secretary, at (563) 589-7178.

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SUMMARY

This summary highlights selected information included in this document and does not contain all of the information that may be important to you. You should read this entire document and its appendices and the other documents to which the parties refer before you decide how to vote with respect to the proposals. In addition, the parties incorporate by reference important business and financial information about MidWestOne into this document. For a description of this information, please see the section entitled "Incorporation of Certain Documents by Reference" beginning on page 189. You may obtain the information incorporated by reference into this document without charge by following the instructions in the section entitled "Where You Can Find More Information" in the forepart of this document. Each item in this summary includes a page reference directing you to a more complete description of that item.

Unless the context otherwise requires, throughout this document, "MidWestOne" refers to MidWestOne Financial Group, Inc., "ATBancorp" refers to ATBancorp and "we," "us" and "our" refer collectively to MidWestOne and ATBancorp. Also, the parties refer to the proposed merger of ATBancorp with and into MidWestOne as the "Merger," the proposed merger of American Trust & Savings Bank and American Bank & Trust Wisconsin with MidWestOne Bank collectively as the "Bank Mergers" and the Agreement and Plan of Merger dated as of August 21, 2018, by and between MidWestOne and ATBancorp as the "Merger Agreement."

The Merger and the Merger Agreement (pages 63 and 113)

The terms and conditions of the Merger are contained in the Merger Agreement, which is attached to this document as Appendix A. The parties encourage you to read the Merger Agreement carefully, as it is the legal document that governs the Merger. Under the terms of the Merger Agreement, ATBancorp will merge with and into MidWestOne with MidWestOne as the surviving corporation.

Merger Consideration (page 114)

In the Merger, each share of ATBancorp common stock issued and outstanding immediately prior to the Effective Time, excluding any Cancelled Shares and any Dissenters' Shares, shall represent the right to receive without interest (i) 117.5500 shares of MidWestOne common stock (the "Per Share Stock Consideration") and (ii) $992.51 (the "Per Share Cash Consideration").

No fractional shares of MidWestOne common stock will be issued to any shareholder of ATBancorp upon completion of the Merger. For each fractional share that would otherwise be issued, MidWestOne will pay an amount in cash (without interest) rounded to the nearest whole cent, determined by multiplying the Closing Acquiror Common Stock Price by the fractional share of MidWestOne common stock to which such former holder would otherwise be entitled. No interest will be paid or accrue on cash payable to holders in lieu of fractional shares.

The ultimate value of the consideration received by ATBancorp shareholders will depend on the trading value of MidWestOne common stock prior to the closing of the Merger. By way of example, the following MidWestOne closing prices would result in the corresponding total Merger consideration and consideration per share of ATBancorp common stock:

MidWestOne Closing Price		Total Merger Consideration		Consideration Per Share of ATBancorp Common Stock

$	28.98	$	154.09 million	$	4,399.11
$	33.25	$	171.67 million	$	4,901.05
$	26.60	$	144.29 million	$	4,119.34

It is currently expected that the former shareholders of ATBancorp as a group will receive shares in the Merger constituting approximately 25.2% of the outstanding shares of the combined company's

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common stock immediately after the Merger. As a result, current shareholders of MidWestOne as a group will own approximately 74.8% of the outstanding shares of the combined company's common stock immediately after the Merger.

Special Dividends (page 108)

The Merger Agreement provides that ATBancorp may pay a special dividend of $907.8452 per share to the ATBancorp shareholders at any time prior to the Closing as determined by ATBancorp. Such dividend represents a portion of the proceeds to ATBancorp from the sale of Heritage Commerce Corp common stock received as consideration in connection with the sale of United American Bank. The special dividend was distributed to ATBancorp shareholders on September 24, 2018.

The Merger Agreement also provides that ATBancorp may pay a special dividend to shareholders based on the net proceeds (if any) received by ATBancorp in connection with the disposition by ATBancorp of the ATB Businesses prior to Closing, as required by the Merger Agreement. The payment of these special dividends by ATBancorp is in addition to the Merger consideration to be paid by MidWestOne. ATBancorp shareholders are not required to take any action to approve or receive these special dividends.

Recommendation of the MidWestOne Board of Directors (page 68)

After careful consideration, the MidWestOne board of directors recommends that MidWestOne shareholders vote "FOR" the MidWestOne merger proposal, "FOR" the MidWestOne stock issuance proposal and "FOR" the MidWestOne adjournment proposal (if necessary or appropriate).

For a more complete description of MidWestOne's reasons for the Merger and the recommendations of the MidWestOne board of directors, please see the section entitled "The Merger—Recommendation of the MidWestOne Board of Directors and Reasons for the Merger" beginning on page 68.

Recommendation of the ATBancorp Board of Directors (page 83)

After careful consideration, the ATBancorp board of directors recommends that ATBancorp shareholders vote "FOR" the ATBancorp merger proposal and "FOR" the ATBancorp adjournment proposal (if necessary or appropriate).

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opposition to or in competition with the Merger. The Voting Agreement will terminate upon the earlier of the consummation of the Merger or the termination of the Merger Agreement in accordance with its terms. Pursuant to the Voting Agreement, approval of the ATBancorp merger proposal is virtually assured. For more information regarding the Voting Agreement, please see the section entitled "The Merger Agreement—Voting Agreement" beginning on page 127.

For a more complete description of ATBancorp's reasons for the Merger and the recommendations of the ATBancorp board of directors, please see the section entitled "The Merger—Recommendation of the ATBancorp Board of Directors and Reasons for the Merger" beginning on page 83.

Opinions of Financial Advisors (pages 70 and 86)

Opinion of MidWestOne's Financial Advisor

In connection with the Merger, MidWestOne's financial advisor, Piper Jaffray & Co. ("Piper Jaffray") delivered a written opinion, dated August 21, 2018, to MidWestOne's board of directors as to the fairness, from a financial point of view and as of the date of the opinion, to MidWestOne of the aggregate Merger consideration in the proposed Merger. The full text of Piper Jaffray's opinion, which describes the procedures followed, assumptions made, matters considered, and qualifications and limitations on the review undertaken by Piper Jaffray in preparing the opinion, is attached as Appendix C to this joint proxy statement/prospectus. This opinion was provided for the information and assistance of the MidWestOne board of directors in connection with its consideration of the Merger. The opinion did not address the merits of MidWestOne's underlying decision to engage in the Merger or the relative merits of the Merger compared to any alternative business strategy or transaction in which MidWestOne might engage. The opinion does not constitute a recommendation to the MidWestOne board of directors or any holder of MidWestOne common stock as to how any MidWestOne board member or such holder should vote with respect to the Merger.

For further information, please see the section entitled "The Merger—Opinion of MidWestOne's Financial Advisor" beginning on page 70.

Opinion of ATBancorp's Financial Advisor

In connection with the Merger, the ATBancorp board of directors received an opinion from Sandler O'Neill & Partners, L.P. ("Sandler O'Neill"), ATBancorp's financial advisor. On August 20, 2018, Sandler O'Neill rendered its oral opinion to the ATBancorp board of directors (which was confirmed in writing by delivery of Sandler O'Neill's written opinion dated August 20, 2018) to the effect that, as of such date, the per share Merger consideration to be received by holders of ATBancorp common stock in the Merger was fair to the holders of ATBancorp common stock, from a financial point of view. The full text of Sandler O'Neill's written opinion is attached as Appendix D to this joint proxy statement/prospectus. ATBancorp shareholders should read the entire opinion for a discussion of, among other things, the assumptions made, matters considered and qualifications and limitations on the review undertaken by Sandler O'Neill in rendering its opinion.

Sandler O'Neill's opinion speaks only as of the date of the opinion. The opinion was directed to ATBancorp's board of directors in connection with its consideration of the Merger Agreement and the Merger and does not constitute a recommendation to any shareholder of ATBancorp as to how any such shareholder should vote at any meeting of shareholders called to consider and vote upon the approval of the Merger Agreement and the Merger. Sandler O'Neill's opinion was directed only to the fairness, from a financial point of view, of the per share Merger consideration to the holders of ATBancorp common stock and does not address the underlying business decision of ATBancorp to engage in the Merger, the form or structure of the Merger or any other transactions contemplated in the Merger Agreement, the relative merits of the Merger as compared to any other alternative

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transactions or business strategies that might exist for ATBancorp or the effect of any other transaction in which ATBancorp might engage.

For further information, please see the section entitled "The Merger—Opinion of ATBancorp's Financial Advisor" beginning on page 86.

MidWestOne Special Meeting of Shareholders (page 53)

The MidWestOne special meeting will be held at 102 South Clinton Street, Summerwill Conference Room, Iowa City, IA 52240, at 10:00 a.m., Central Time, on Friday, January 11, 2019. At the MidWestOne special meeting, MidWestOne shareholders will be asked to approve the MidWestOne merger proposal, the MidWestOne stock issuance proposal and the MidWestOne adjournment proposal (if necessary or appropriate).

MidWestOne's board of directors has fixed the close of business on November 26, 2018 as the record date for determining the holders of MidWestOne common stock entitled to receive notice of and to vote at the MidWestOne special meeting. As of the MidWestOne record date, there were 12,222,145 shares of MidWestOne common stock outstanding and entitled to vote at the MidWestOne special meeting held by approximately 425 holders of record. Each share of MidWestOne common stock entitles the holder to one vote on each proposal to be considered at the MidWestOne special meeting. As of the MidWestOne record date, directors and executive officers of MidWestOne owned and were entitled to vote 392,994 shares of MidWestOne common stock, representing approximately 3.2% of the shares of MidWestOne common stock outstanding on that date. MidWestOne currently expects that MidWestOne's directors and executive officers will vote their shares in favor of the proposals to be presented at the special meeting, although none of them has entered into any agreements obligating them to do so. As of the MidWestOne record date, ATBancorp beneficially held no shares of MidWestOne common stock.

Approval of the MidWestOne merger proposal requires the affirmative vote of a majority of the votes entitled to be cast on the proposal. Approval of the MidWestOne stock issuance proposal and the MidWestOne adjournment proposal requires the affirmative vote of a majority of the shares of MidWestOne common stock represented in person or by proxy at the MidWestOne special meeting and entitled to vote on such proposals.

ATBancorp Special Meeting of Shareholders (page 59)

The ATBancorp special meeting will be held at American Trust & Savings Bank, 895 Main Street, Dubuque, lowa 52001, at 11:00 a.m., Central Time, on Thursday, January 10, 2019. At the ATBancorp special meeting, ATBancorp shareholders will be asked to approve the ATBancorp merger proposal and, if necessary or appropriate, the ATBancorp adjournment proposal.

ATBancorp's board of directors has fixed the close of business on November 15, 2018 as the record date for determining the holders of ATBancorp common stock entitled to receive notice of and to vote at the ATBancorp special meeting. As of the ATBancorp record date, there were 35,028 shares of ATBancorp common stock outstanding and entitled to vote at the ATBancorp special meeting held by 18 holders of record. Each share of ATBancorp common stock entitles the holder to one vote on each proposal to be considered at the ATBancorp special meeting. As of the ATBancorp record date, directors and executive officers of ATBancorp owned and were entitled to vote 23,323 shares of ATBancorp common stock, representing approximately 66.6% of the shares of ATBancorp common stock outstanding on that date. In connection with entering into the Merger Agreement, MidWestOne entered into the Voting Agreement with certain holders of ATBancorp common stock. The shareholders that are party to the Voting Agreement beneficially own in the aggregate 62.41% of the outstanding shares of ATBancorp common stock. The Voting Agreement requires that the shareholders party thereto vote all of their shares of ATBancorp common stock in favor of the Merger and against

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alternative acquisition proposals. The Voting Agreement will terminate upon the earlier of the consummation of the Merger or the termination of the Merger Agreement in accordance with its terms. As of the ATBancorp record date, MidWestOne beneficially held no shares of ATBancorp's common stock.

Approval of the ATBancorp merger proposal requires the affirmative vote of a majority of the shares of ATBancorp common stock represented in person or by proxy at the ATBancorp special meeting and entitled to vote on this proposal. Pursuant to the Voting Agreement, approval of the ATBancorp merger proposal is virtually assured. Approval of the ATBancorp adjournment proposal requires the affirmative vote of a majority of the shares of ATBancorp common stock represented in person or by proxy at the ATBancorp special meeting and entitled to vote on this proposal.

ATBancorp's Directors and Executive Officers Have Certain Interests in the Merger (page 102)

ATBancorp's executive officers and directors have interests in the Merger that are different from, or in addition to, the interests of ATBancorp's shareholders generally. Such interests include change in control agreements, deferred compensation agreements and salary continuation agreements with certain executive officers and the right to indemnification and insurance coverage following the consummation of the Merger. The members of the ATBancorp board of directors were aware of and considered these interests, among other matters, when they approved the Merger Agreement and recommended that ATBancorp shareholders approve the ATBancorp merger proposal. These interests are described in more detail under the section entitled "The Merger—Interests of ATBancorp Directors and Executive Officers in the Merger" beginning on page 101.

Management and Board of Directors of MidWestOne after the Merger (page 101)

Pursuant to the Merger Agreement, MidWestOne shall take all appropriate action, subject to and in accordance with MidWestOne's articles of incorporation, bylaws, board policies and applicable legal requirements, to appoint two individuals to MidWestOne's Board who are designated by ATBancorp, by a vote of at least 75% of ATBancorp's Board, and are agreeable to MidWestOne. One of the individuals so designated by ATBancorp must not be a current officer, director or shareholder of ATBancorp and must have an established history in the Dubuque, Iowa business community (the "Dubuque Community Director"). One individual will be designated to serve as a Class I director and the other individual will be designated to serve as a Class II director, with the class assignments determined by MidWestOne, in each case, effective immediately upon the effectiveness of the Merger.

Regulatory Approvals Required for the Merger (page 106)

Completion of the Merger and the Bank Mergers are subject to various regulatory approvals, including approval or waiver from the Board of Governors of the Federal Reserve System (the "Federal Reserve Board"), and the approvals of the Federal Deposit Insurance Corporation (the "FDIC"), the Iowa Division of Banking (the "IDB") and the Wisconsin Department of Financial Institutions (the "WDFI"). The parties have filed notices and applications to obtain the necessary regulatory approvals or waivers, as applicable of the Federal Reserve Board, FDIC, the IDB and WDFI. On October 11, 2018, the Federal Reserve Board informed the parties that its legal division would not recommend that the Federal Reserve Board require the filing of an application by MidWestOne in connection with the Merger, on October 29, 2018, the FDIC provided the parties with its approval of the Bank Mergers and on November 14, 2018, the WDFI provided the parties with its approval of the acquisition of ABTW by MidWestOne. The regulatory approvals to which completion of the Merger and Bank Mergers are subject are described in more detail under the section entitled "The Merger—Regulatory Approvals Required for the Merger" beginning on page 106.

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Conditions to Consummation of the Merger (page 124)

The respective obligation of each party to effect the Merger is subject to the satisfaction or written waiver at or prior to the Effective Time of each of the following conditions:

•: all actions and approvals by governmental authorities and certain other specified actions and approvals shall have been taken and obtained and shall remain in full force and effect and all statutory waiting periods shall have expired or been terminated and no such requisite regulatory approval shall have imposed a restriction or condition on, or requirement of, such approval that would, after the Effective Time, reasonably be expected by the board of directors of MidWestOne to materially restrict or burden, or impair in any material respect the benefits of the contemplated transactions to, or require a materially burdensome modification of, the businesses, activities, governance, legal structure, capital structure, compensation or fee arrangements of MidWestOne or its subsidiaries;
•: ATBancorp and MidWestOne shall have each obtained the approval of the Merger Agreement and the transactions contemplated thereby by their respective shareholders;
•: no order issued by any court or governmental agency or other legal restraint or prohibition preventing the consummation of the Merger or the other transactions contemplated by the Merger Agreement shall be in effect, and no legal requirement shall have been enacted, entered, promulgated or enforced by any governmental authority which prohibits or makes illegal consummation of the Merger;
•: the S-4 registration statement of which this joint proxy statement/prospectus is a part shall have become effective under the Securities Act, and no stop orders suspending such effectiveness shall be in effect, and no proceeding shall have been commenced or be pending or threatened for such purpose; and
•: MidWestOne shall have filed with the Nasdaq Stock Market, LLC a notification form for the listing of all shares of MidWestOne common stock to be delivered in the Merger, and the Nasdaq Stock Market, LLC shall not have objected to the listing of such shares.

ATBancorp's obligation to effect the Merger is also subject to the fulfillment or waiver of the following conditions:

•: the accuracy of the representations and warranties of MidWestOne set forth in the Merger Agreement as of the date of the Merger Agreement and as of the Closing Date as though made at and as of the Closing Date (unless any such representation or warranty is made only as of a specific date, in which case as of such specific date);
•: the performance by MidWestOne in all material respects of all covenants and obligations to be performed or complied with by it under the terms of the Merger Agreements on or prior to the Closing Date;
•: the receipt of a certificate, signed on behalf of MidWestOne by an executive officer, certifying that certain of the conditions to ATBancorp's obligation to complete the Merger have been satisfied;
•: receipt by ATBancorp of a written opinion of its tax accountants to the effect that the Merger will constitute a "reorganization" within the meaning of Section 368(a) of the Internal Revenue Code; and
•: from the date of the Merger Agreement to closing, no material adverse effect shall have occurred with respect to MidWestOne or any of its subsidiaries.

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MidWestOne's obligation to effect the Merger is also subject to the satisfaction or waiver of the following conditions:

•: the accuracy of the representations and warranties of ATBancorp set forth in the Merger Agreement as of the date of the Merger Agreement and as of the Closing Date as though made at and as of the Closing Date (unless any such representation or warranty is made only as of a specific date, in which case as of such specific date);
•: the performance by ATBancorp in all material respects of all covenants and obligations to be performed or complied with by it under the terms of the Merger Agreements on or prior to the Closing Date, and ATBancorp's obtainment of and provision to MidWestOne of copies of all ATBancorp required approvals;
•: the receipt of a certificate, signed on behalf of ATBancorp by an executive officer certifying that certain of the conditions to MidWestOne's obligation to complete the Merger have been satisfied;
•: the receipt by MidWestOne of a written opinion of its counsel to the effect that the Merger will constitute a "reorganization" within the meaning of Section 368(a) of the Internal Revenue Code;
•: from the date of the Merger Agreement to closing, no material adverse effect shall have occurred with respect to ATBancorp or any of its subsidiaries;
•: ATBancorp shall have satisfied all of the conditions necessary to complete the redemption of all of ATBancorp's subordinated debentures bearing a fixed annual rate per annum of 6.50% and maturing September 15, 2021, in the aggregate principal amount of $9,600,000;
•: the receipt of duly executed copies of the Lock-Up Agreements, which have been delivered;
•: the total number of outstanding shares of ATBancorp common stock that have duly exercised their appraisal rights shall not exceed 7.5% of the outstanding shares of ATBancorp common stock;
•: ATBancorp shall have completed the disposition of an aircraft and the ATB Businesses; and
•: ATBancorp shall have delivered to MidWestOne all other instruments and documents that MidWestOne may reasonably request to effectuate the transactions contemplated in the Merger Agreement.

Acquisition Proposals (page 123)

Under the terms of the Merger Agreement, ATBancorp has agreed that it shall not, and it shall cause its subsidiaries and its subsidiaries' representatives not to, directly or indirectly:

•: initiate, solicit, encourage or knowingly facilitate any inquiries or proposals with respect to, or engage in any discussions or negotiations concerning, or provide any information, including any confidential or nonpublic information, or data to, or have any discussions with, any person relating to, any proposal or offer that constitutes, or could reasonably be expected to lead to, an acquisition proposal; or
•: approve, recommend or enter into, or propose to approve, recommend or enter into, any letter of intent or similar document, memorandum of understanding, agreement, commitment, or agreement in principle with respect to an acquisition proposal.

ATBancorp has agreed to promptly notify MidWestOne of any such unsolicited acquisition proposal or of any request for information relating to ATBancorp or any of its subsidiaries that is

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reasonably likely to lead to or that contemplates an acquisition proposal, including the identity of the proposed acquirer and the material terms and conditions of such proposal.

Termination of the Merger Agreement (page 126)

MidWestOne and ATBancorp may mutually agree at any time to terminate the Merger Agreement without completing the Merger. The Merger Agreement may also be terminated by MidWestOne or ATBancorp if:

•: the other party breaches or fails to perform any of its representations, warranties, covenants or agreements set forth in the Merger Agreement, which breach or failure to perform, either individually or together with other such breaches, in the aggregate, if occurring or continuing on the date on which the Closing would otherwise occur would result in the failure of any of the conditions to closing and such breach or failure to perform has not been or cannot be cured on or prior to the earlier of two business days prior to June 30, 2019 (the "Termination Date") and 30 days following written notice to the party committing such breach, making such untrue representation and warranty or failing to perform;
•: (i) any regulatory authority that must grant a requisite regulatory approval has denied approval of any of the contemplated transactions and such denial has become final and nonappealable; (ii) any application, filing or notice for a requisite regulatory approval has been withdrawn at the request or recommendation of the applicable regulatory authority; or (iii) if (A) ATBancorp does not obtain the approval of the Merger Agreement and the transactions contemplated thereby by its shareholders or (B) MidWestOne does not obtain the approval of the issuance of shares of common stock in connection with the Merger by its shareholders, each following the ATBancorp shareholders' meeting or the MidWestOne shareholders' meeting, respectively; provided, however, that the right to terminate shall not be available to a party whose failure to fulfill any of its obligations under the Merger Agreement has been the cause of or resulted in the occurrence of any event described in clause (iii) above and such failure constitutes a material breach of the Merger Agreement;
•: the Effective Time has not occurred at or before the Termination Date; provided, however, that the right to terminate shall not be available to any party whose failure to fulfill any of its obligations under the Merger Agreement has been the cause of, or has resulted in, the failure of the Effective Time to occur on or before such date and such failure constitutes a material breach of the Merger Agreement; or
•: any court of competent jurisdiction or other regulatory authority shall have issued a judgment, order, injunction, rule or decree or taken any other action restraining, enjoining or otherwise prohibiting any of the contemplated transactions and such judgment, order, injunction, rule, decree or other action shall have become final and nonappealable.

Further, MidWestOne may terminate the Merger Agreement if:

•: ATBancorp makes a Company Adverse Recommendation (as defined in the Merger Agreement) or if ATBancorp materially breaches certain of its other obligations under the Merger Agreement relating to holding a shareholders' meeting or acquisition proposals; and
•: if certain events, conditions or circumstances require further investigation, remedial or cleanup action under certain environmental laws and such expenditures are reasonably expected to exceed $10,000,000.

Further, ATBancorp may terminate the Merger Agreement if:

•: if both of the following conditions are satisfied on the Determination Date, such termination to be effective on the 10th day following the Determination Date: (i) the Final Acquiror Market

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Value is less than $26.60; and (ii) the number obtained by dividing the Final Acquiror Market Value by $33.25 shall be less than the Index Ratio multiplied by 0.80; subject, however, to MidWestOne's exercise of its option to increase the per share stock consideration payable to ATBancorp shareholders, in which event no termination shall occur; or

•: under certain circumstances it receives an acquisition proposal that would constitute a superior proposal.

For more information, please see the section entitled "The Merger Agreement—Termination of the Merger Agreement" beginning on page 126.

Termination Fees (page 127)

The Merger Agreement also provides that upon termination of the Merger Agreement by either MidWestOne under certain circumstances, including ATBancorp failing to convene a meeting of its shareholders to consider and approve the Merger Agreement and the transactions contemplated thereby, or by ATBancorp by accepting a superior proposal, ATBancorp will be obligated to pay MidWestOne a termination fee of $7.6 million, plus the aggregate amount of MidWestOne's expenses incurred in connection with the Merger, provided that the aggregate amount of such expenses shall not exceed $1,000,000.

Voting Agreement (page 127)

In connection with entering into the Merger Agreement, MidWestOne entered into the Voting Agreement with certain holders of ATBancorp common stock. The shareholders that are party to the Voting Agreement beneficially own in the aggregate 62.41% of the outstanding shares of ATBancorp common stock. The Voting Agreement requires that the shareholders party thereto vote all of their shares of ATBancorp common stock in favor of the Merger and against any proposal made in opposition to or in competition with the Merger. The Voting Agreement will terminate upon the earlier of the consummation of the Merger or the termination of the Merger Agreement in accordance with its terms. Pursuant to the Voting Agreement, approval of the ATBancorp merger proposal is virtually assured. For more information regarding the Voting Agreement, please see the section entitled "The Merger Agreement—Voting Agreement" beginning on page 127.

Lock-Up Agreements (page 128)

In connection with entering into the Merger Agreement, MidWestOne entered into Lock-Up Agreements ("Lock-Up Agreements") with certain holders of ATBancorp common stock. The shareholders that are party to the Lock-Up Agreements beneficially own in the aggregate 62.41% of the outstanding shares of ATBancorp common stock. The Lock-Up Agreements require that the shareholders that are parties thereto, during the period commencing as of the date on which the Effective Time occurs and expiring 180 days thereafter, not directly or indirectly, with certain exceptions, take any action to: (i) offer, sell, contract to sell, sell any option, warrant or contract to purchase, purchase any option, warrant or contract to sell, transfer, pledge or otherwise dispose of, or enter into any transaction which is designed to, or might reasonably be expected to, result in the disposition (whether by actual disposition or effective economic disposition or otherwise) of any shares of MidWestOne common stock whether owned as of the time of the Lock-Up Agreements or subsequently acquired, including shares of MidWestOne common stock received by the shareholder pursuant to the Merger Agreement; (ii) enter into any swap or other derivative transaction that transfers to another, in whole or in part, any of the economic benefits or risks of ownership of MidWestOne common stock, whether any such transaction is to be settled by delivery of MidWestOne common stock or other securities, in cash or otherwise; or (iii) publicly disclose an intention to effect any transaction contemplated by clause (i) or (ii). If the Merger Agreement is terminated without the

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consummation of the Merger, the Lock-Up Agreements will automatically terminate. For more information regarding the Lock-Up Agreements, please see the section entitled "The Merger Agreement—Lock-Up Agreements" beginning on page 128.

Material U.S. Federal Income Tax Consequences of the Merger (page 129)

The Merger is intended to qualify as a reorganization within the meaning of Section 368(a) of the Internal Revenue Code. Assuming the Merger qualifies as a reorganization, a shareholder of ATBancorp generally will not recognize any gain or loss upon receipt of MidWestOne common stock in exchange for ATBancorp common stock in the Merger, and will recognize gain (but not loss) in an amount not to exceed any cash received as part of the Merger consideration (except with respect to any cash received in lieu of a fractional share of MidWestOne common stock, as discussed below under "Material U.S. Federal Income Tax Consequences of the Merger—Cash Received In Lieu of a Fractional Share of MidWestOne Common Stock"). It is a condition to the completion of the Merger that MidWestOne and ATBancorp receive written opinions from their tax counsel or tax accountants to the effect that the Merger will qualify as a reorganization within the meaning of Section 368(a) of the Internal Revenue Code.

Tax matters are complicated and the tax consequences of the Merger to each ATBancorp shareholder may depend on such shareholder's particular facts and circumstances. ATBancorp shareholders are urged to consult their tax advisors to understand fully the tax consequences to them of the Merger. For more information, please see the section entitled "Material U.S. Federal Income Tax Consequences of the Merger" beginning on page 129.

Comparison of Shareholders' Rights (page 134)

The rights of ATBancorp shareholders who continue as MidWestOne shareholders after the Merger will be governed by the articles of incorporation and bylaws of MidWestOne rather than by the articles of incorporation and bylaws of ATBancorp. For more information, please see the section entitled "Comparison of Shareholders' Rights" beginning on page 134.

The Parties (pages 141 and 142)

MidWestOne Financial Group, Inc.
102 South Clinton Street
Iowa City, Iowa 52240
Phone: (319) 356-5800

MidWestOne is an Iowa corporation incorporated in 1983, is a bank holding company registered under the Bank Holding Company Act of 1956, as amended (the "BHC Act") and a financial holding company under the Gramm-Leach-Bliley Act of 1999. As of June 30, 2018, MidWestOne had consolidated total assets of $3.276 billion, total loans held for investment of $2.3 billion, deposits of $2.6 billion and shareholders' equity of $346 million.

ATBancorp
895 Main Street
Dubuque, Iowa 52001
Phone: (563) 582-1841

ATBancorp is an Iowa corporation incorporated in 1985 and a bank holding company registered under the BHC Act. As of June 30, 2018, ATBancorp had consolidated total assets of $1.4 billion, total loans held for investment of $1.1 billion, deposits of $1.1 billion and shareholders' equity of $146.0 million.

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Risk Factors (page 44)

Before voting at the MidWestOne special meeting or ATBancorp special meeting, you should carefully consider all of the information contained in or incorporated by reference into this document, including the risk factors set forth in the section entitled "Risk Factors" beginning on page 44 or described in MidWestOne's Annual Report on Form 10-K for the year ended on December 31, 2017 and other reports filed with the SEC, which are incorporated by reference into this document. Please see "Where You Can Find More Information" in the forepart of this document and "Incorporation of Certain Documents by Reference" beginning on page 189.

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CERTAIN FINANCIAL INFORMATION REGARDING MIDWESTONE AND ATBANCORP

Selected Financial Information of MidWestOne

The following table summarizes consolidated financial results achieved by MidWestOne for the periods and at the dates indicated and should be read in conjunction with MidWestOne's consolidated financial statements and the notes to the consolidated financial statements contained in reports that MidWestOne has previously filed with the SEC. Historical financial information for MidWestOne can be found in its Annual Report on Form 10-K for the year ended December 31, 2017 and its Quarterly Report on Form 10-Q for the nine-month period ended September 30, 2018. Please see the section entitled "Where You Can Find More Information" for instructions on how to obtain the information

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that has been incorporated by reference. You should not assume the results of operations for past years indicate results for any future period.

	As of and for the Years Ended December 31,

	2017			2016			2015			2014			2013
	(dollars in thousands except per share)
Consolidated Statement of Income Data:
Interest income	$	119,320		$	112,328		$	100,700		$	64,404		$	66,094
Interest expense		15,145			12,722			10,648			9,551			12,132

Net interest income		104,175			99,606			90,052			54,853			53,962
Provision for loan losses		17,334			7,983			5,132			1,200			1,350

Net interest income after provision for loan losses		86,841			91,623			84,920			53,653			52,612
Noninterest income		22,370			23,434			21,193			15,313			14,728
Noninterest expense		80,136			87,806			73,176			43,413			42,087

Income before income taxes		29,075			27,251			32,937			25,553			25,253
Income tax expense		10,376			6,860			7,819			7,031			6,646

Net income	$	18,699		$	20,391		$	25,118		$	18,522		$	18,607



Common Share Data:
Basic earnings per share	$	1.55		$	1.78		$	2.42		$	2.20		$	2.19
Diluted earnings per share	$	1.55		$	1.78		$	2.42		$	2.19		$	2.18
Cash dividends declared per share	$	0.67		$	0.64		$	0.60		$	0.58		$	0.50
Book value per share	$	27.85		$	26.71		$	25.96		$	23.07		$	20.99
Weighted average shares outstanding-basic		12,038			11,430			10,363			8,405			8,478
Weighted average shares outstanding-diluted		12,063			11,456			10,391			8,433			8,525
Shares outstanding at end of period		12,220			11,436			11,409			8,356			8,482
Consolidated Balance Sheet Data:
Cash and cash equivalents	$	50,972		$	43,228		$	47,097		$	23,409		$	24,890
Investment securities	$	643,279		$	645,910		$	545,664		$	526,466		$	531,186
Loans held for investment, net of unearned income	$	2,286,695		$	2,165,143		$	2,151,942		$	1,132,519		$	1,088,412
Allowance for loan losses	$	28,059		$	21,850		$	19,427		$	16,363		$	16,179
Total assets	$	3,212,271		$	3,079,575		$	2,979,975		$	1,800,302		$	1,755,218
Total deposits	$	2,605,319		$	2,480,448		$	2,463,521		$	1,408,542		$	1,374,942
Total borrowings and debt	$	248,522		$	274,063		$	202,050		$	186,693		$	189,029
Shareholders' equity	$	340,304		$	305,456		$	296,178		$	192,731		$	178,016
Average Balance Sheet Data:
Average assets	$	3,097,496		$	2,993,875		$	2,773,095		$	1,760,776		$	1,756,344
Average earning assets	$	2,853,830		$	2,747,493		$	2,541,681		$	1,669,130		$	1,667,251
Average shareholders' equity	$	334,966		$	304,670		$	255,307		$	186,375		$	175,666
Financial Ratios:
Return on average assets(a)		0.60	%		0.68	%		0.91	%		1.05	%		1.06	%
Return on average equity(b)		5.58	%		6.69	%		9.84	%		9.94	%		10.59	%
Net interest margin (FTE)(c)		3.83	%		3.80	%		3.71	%		3.53	%		3.46	%
Efficiency ratio, adjusted(d)		58.64	%		66.43	%		61.36	%		58.71	%		57.11	%

(a): Return on average assets is determined by dividing net income by average assets.
(b): Return on average equity is determined by dividing net income by average shareholders' equity.

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(c): Net interest margin (FTE) is determined by dividing net interest income, reported on a fully tax-equivalent basis assuming a 35% tax rate, by average earning assets. As presented, net interest margin (FTE) is a non-GAAP financial measure, and the most directly comparable GAAP measure is net interest margin. MidWestOne believes that the presentation of net interest margin on a tax equivalent basis provides useful information to investors regarding its results of operations because it provides a more accurate representation of MidWestOne's actual net interest margin. A reconciliation of net interest margin is presented below.
(d): As presented, the adjusted efficiency ratio is a non-GAAP financial measure. MidWestOne believes that the presentation of the adjusted efficiency ratio provides useful information to investors regarding its results of operations because it provides additional insights as to MidWestOne's current performance. A reconciliation is presented below.

	As of and for the Years Ended December 31,

	2017			2016			2015			2014			2013
Nonperforming Assets
Nonperforming assets to total assets		0.81	%		0.99	%		0.68	%		0.83	%		0.89	%
Nonperforming loans to loans held for investment, net of unearned income		1.04	%		1.31	%		0.54	%		1.15	%		1.27	%
Allowance for loan losses to loans held for investment, net of unearned income		1.23	%		1.01	%		0.90	%		1.44	%		1.49	%
Allowance for loan losses to nonperforming loans		117.59	%		76.76	%		168.52	%		125.67	%		117.44	%
Net loan charge-offs to average loans		0.51	%		0.26	%		0.11	%		0.09	%		0.11	%

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The following tables provide a reconciliation of the non-GAAP measures to the most comparable GAAP equivalent, in each case as of the dates presented.

	As of and for the Years Ended December 31,

	2017			2016			2015			2014			2013
	(dollars in thousands)
Net Interest Margin Tax Equivalent Adjustment
Net interest income	$	104,175		$	99,606		$	90,052		$	54,853		$	53,962
Plus tax equivalent adjustment:(1)
Loans		1,730			1,692			1,293			1,157			963
Investment securities		3,297			3,023			2,898			2,880			2,795

Tax equivalent net interest income(1)	$	109,202		$	104,321		$	94,243		$	58,890		$	57,720

Average interest-earning assets	$	2,853,830		$	2,747,493		$	2,541,681		$	1,669,130		$	1,667,251

Net Interest Margin (FTE)		3.83	%		3.80	%		3.71	%		3.53	%		3.46	%



Net Interest Margin		3.65	%		3.63	%		3.54	%		3.29	%		3.24	%



Operating Expense
Total noninterest expense	$	80,136		$	87,806		$	73,176		$	43,413		$	42,087
Less: Amortization of intangibles		(3,125	)		(3,970	)		(3,271	)		(547	)		(663	)

Operating expense	$	77,011		$	83,836		$	69,905		$	42,866		$	41,424

Operating Revenue
Tax-equivalent net interest income(1)	$	109,202		$	104,321		$	94,243		$	58,890		$	57,720
Plus: Noninterest income		22,370			23,434			21,193			15,313			14,728
Less: Gain on sale or call of available for sale securities		188			464			1,011			1,227			65
Gain on sale or call of held to maturity securities		53			—			—			—			—
Gain (loss) on sale of premises and equipment		2			(44	)		(29	)		(1	)		(3	)
Other gain (loss)		11			1,133			527			(37	)		(146	)

Operating Revenue	$	131,318		$	126,202		$	113,927		$	73,014		$	72,532

Efficiency Ratio, adjusted		58.64	%		66.43	%		61.36	%		58.71	%		57.11	%



Efficiency Ratio		63.33	%		71.36	%		65.78	%		61.87	%		61.27	%

(1): Computed on a tax equivalent basis, assuming a federal income tax rate of 35%.

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	As of and for the Nine Months Ended September 30,

	2018			2017
	(dollars in thousands except per share)
Consolidated Statement of Income Data:
Interest income	$	95,425		$	88,782
Interest expense		16,170			11,018

Net interest income		79,255			77,764
Provision for loan losses		4,050			6,665

Net interest income after provision for loan losses		75,205			71,099
Noninterest income		17,143			16,836
Noninterest expense		63,700			60,043

Income before income taxes		28,648			27,892
Income tax expense		5,921			7,603

Net income	$	22,727		$	20,289



Common Share Data:
Basic earnings per share	$	1.86		$	1.69
Diluted earnings per share	$	1.86		$	1.69
Cash dividends declared per share	$	0.585		$	0.50
Book value per share	$	28.57		$	28.36
Weighted average shares outstanding-basic		12,221			11,978
Weighted average shares outstanding-diluted		12,238			12,000
Shares outstanding at end of period		12,221			12,219
Consolidated Balance Sheet Data:
Cash and cash equivalents	$	53,379		$	50,972
Investment securities	$	602,296		$	643,279
Loans held for investment, net of unearned income	$	2,377,649		$	2,286,695
Allowance for loan losses	$	31,278		$	28,059
Total assets	$	3,267,965		$	3,212,271
Total deposits	$	2,632,259		$	2,605,319
Total borrowings and debt	$	263,593		$	248,522
Shareholders' equity	$	349,189		$	340,304
Average Balance Sheet Data:
Average assets	$	3,258,283		$	3,102,348
Average earning assets	$	3,004,892		$	2,858,715
Average shareholders' equity	$	348,131		$	344,961
Financial Ratios:
Return on average assets(a)		0.94	%		0.88	%
Return on average equity(b)		8.84	%		8.20	%
Net interest margin (FTE)(c)		3.64	%		3.85	%
Efficiency ratio, adjusted(d)		63.30	%		58.78	%
Nonperforming Assets
Nonperforming assets to total assets		0.89	%		0.87	%
Nonperforming loans to loans held for investment, net of unearned income		1.20	%		1.14	%
Allowance for loan losses to loans held for investment, net of unearned income		1.32	%		1.17	%
Allowance for loan losses to nonperforming loans		109.92	%		102.40	%
Net loan charge-offs to average loans		0.05	%		0.12	%

(a): Return on average assets is determined by dividing net income by average assets.
(b): Return on average equity is determined by dividing net income by average shareholders' equity.

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(c): Net interest margin (FTE) is determined by dividing net interest income, reported on a fully tax-equivalent basis assuming a federal income tax rate of 21% for 2018, and 35% for 2017, by average earning assets. As presented, net interest margin (FTE) is a non-GAAP financial measure, and the most directly comparable GAAP measure is net interest margin. MidWestOne believes that the presentation of net interest margin on a tax equivalent basis provides useful information to investors regarding its results of operations because it provides a more accurate representation of MidWestOne's actual net interest margin. A reconciliation of net interest margin is presented below.
(d): As presented, the adjusted efficiency ratio is a non-GAAP financial measure. MidWestOne believes that the presentation of the adjusted efficiency ratio provides useful information to investors regarding its results of operations because it provides additional insights as to MidWestOne's current performance. A reconciliation is presented below.

The following tables provide a reconciliation of the non-GAAP measures to the most comparable GAAP equivalent, in each case as of the dates presented.

	As of and for the Nine Months Ended September 30,

	2018			2017
	(dollars in thousands)
Net Interest Margin Tax Equivalent Adjustment
Net interest income	$	79,255		$	77,764
Plus tax equivalent adjustment:(1)
Loans		769			1,251
Investment securities		1,167			2,491

Tax equivalent net interest income(1)	$	81,191		$	81,506

Average interest-earning assets	$	2,988,193		$	2,831,864

Net Interest Margin (FTE)		3.64	%		3.85	%



Net Interest Margin		3.55	%		3.67	%



Operating Expense
Total noninterest expense	$	63,700		$	60,043
Less: Amortization of intangibles		(1,793	)		(2,412	)

Operating expense	$	61,907		$	57,631

Operating Revenue
Tax-equivalent net interest income(1)	$	81,191		$	81,506
Plus: Noninterest income		17,143			16,836
Less: Gain on sale or call of debt securities		(197	)		(239	)
Other (gain) loss		(338	)		(66	)

Operating Revenue	$	97,799		$	98,037

Efficiency Ratio, adjusted		63.30	%		58.78	%



Efficiency Ratio		66.08	%		63.47	%

(1): Computed on a tax equivalent basis, assuming a federal income tax rate of 21% for 2018, and 35% for 2017.

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Unaudited Pro Forma Condensed Combined Financial Information

The following is the unaudited pro forma combined financial data for MidWestOne and ATBancorp, giving effect to the Merger. The unaudited pro forma combined balance sheet as of September 30, 2018 gives effect to the Merger as if it occurred on that date. The unaudited pro forma combined income statement for the year ended December 31, 2017 and for the nine months ended September 30, 2018 gives effect to the Merger as if it occurred as of the beginning of each period

The unaudited pro forma combined financial statements have been prepared using the acquisition method of accounting for business combinations under accounting principles generally accepted in the United States of America ("GAAP"). MidWestOne is the acquirer for accounting purposes. Certain immaterial reclassifications have been made to the historical financial statements of ATBancorp to conform to the presentation in MidWestOne's financial statements.

A final determination of the fair values of ATBancorp's assets and liabilities, which cannot be made prior to the completion of the Merger, will be based on the actual net tangible and intangible assets of ATBancorp that exist as of the date of completion of the transaction. Consequently, fair value adjustments to assets acquired (including identifiable intangible assets) and liabilities assumed could change significantly from those in the unaudited pro forma combined financial statements presented herein and could result in a material change in amortization of acquired intangible assets. In addition, the value of the final purchase price of the Merger will be based on the closing price of MidWestOne's common stock immediately preceding the Closing Date. A closing price of MidWestOne common stock of $33.31 was used for purposes of presenting the unaudited pro forma combined balance sheet at September 30, 2018.

In connection with the plan to integrate the operations of MidWestOne and ATBancorp following the completion of the Merger, MidWestOne anticipates that it will incur nonrecurring charges, such as costs associated with systems implementation, severance and other costs related to exit or disposal activities. MidWestOne is not able to determine the timing, nature and amount of these charges as of the date of this joint proxy statement/prospectus. However, these charges will affect the results of operations of MidWestOne and ATBancorp, as well as those of the combined company following the completion of the Merger, in the period in which they are recorded. The unaudited pro forma combined income statements do not include the effects of the non-recurring costs associated with any restructuring or integration activities resulting from the Merger, as they are nonrecurring in nature and not factually supportable at this time.

The Merger Agreement provides that ATBancorp may pay a special dividend of $907.8452 per share to the ATBancorp shareholders at a time prior to the Closing determined by ATBancorp. Such dividend represents a portion of the proceeds to ATBancorp from the sale of Heritage Commerce Corp common stock received by ATBancorp as consideration in connection with the sale, completed on May 4, 2018, of United American Bank. ATBancorp shareholders are not required to take any action to approve or receive this special dividend. The payment of this special dividend by ATBancorp is in addition to the Merger consideration to be paid by MidWestOne. The unaudited pro forma combined financial statements reflect the special dividend of $907.8452 per share that was distributed to ATBancorp shareholders on September 24, 2018. In addition, the unaudited pro forma combined financial statements reflect the removal of historical income and expense amounts recognized by ATBancorp from the operations of United American Bank.

The Merger Agreement also provides that, prior to Closing, ATBancorp dispose of an aircraft and the ATB Businesses; the assets and liabilities of the ATB Businesses consist primarily of customers, contracts and employees. ATBancorp may pay a special dividend to ATBancorp shareholders based on the net proceeds (if any) received by ATBancorp in connection with the disposition of the ATB Businesses, prior to Closing. ATBancorp shareholders are not required to take any action to approve or receive this special dividend. The payment of this special dividend by ATBancorp is in addition to the

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Merger consideration to be paid by MidWestOne. The unaudited pro forma combined financial statements reflect the removal of historical estimated amounts attributed to the ATB Businesses. The unaudited pro forma combined financial statements do not, however, include the special dividend to shareholders based on the net proceeds received in connection with the disposition of the ATB Businesses, as MidWestOne is not able to determine the amount of such dividend as of the date of this joint proxy statement/prospectus.

The actual amounts recorded as of the completion of the Merger may differ materially from the information presented in these unaudited pro forma combined financial statements as a result of, among other things:

•: changes in the trading price for MidWestOne's common stock;
•: capital used or generated in ATBancorp's operations between the signing of the Merger Agreement and completion of the Merger;
•: changes in ATBancorp's net assets that occur prior to the completion of the Merger;
•: changes in the fair values of ATBancorp's assets and liabilities; and
•: the actual financial results of the combined company.

The unaudited pro forma combined financial statements are provided for informational purposes only. The unaudited pro forma combined financial statements are not necessarily, and should not be assumed to be, an indication of the results that would have been achieved had the transaction been completed as of the dates indicated or that may be achieved in the future. The preparation of the unaudited pro forma combined financial statements and related adjustments required management to make certain assumptions and estimates. The unaudited pro forma combined financial information is based on, and should be read together with, the historical consolidated financial statements and related notes of MidWestOne incorporated into this document by reference from its Annual Report on Form 10-K for the year ended December 31, 2017 and its Quarterly Reports on Form 10-Q for the quarters ended March 31, 2018, June 30, 2018, and September 30, 2018, ATBancorp's audited consolidated financial statements and accompanying notes as of and for the year ended December 31, 2017, ATBancorp's unaudited consolidated financial statements and accompanying notes as of and for the three and nine months ended March 31, 2018 and September 30, 2018 and ATBancorp's Management's Discussion and Analysis of Financial Condition and Results of Operations for the year ended December 31, 2017 and the three and nine months ended March 31, 2018 and September 30, 2018 included elsewhere in this joint proxy statement/prospectus.

The unaudited pro forma condensed combined balance sheet as of September 30, 2018 presents the consolidated financial position giving pro forma effect to the following transactions as if they had occurred as of September 30, 2018:

•: the completion of MidWestOne's acquisition of ATBancorp, including the issuance of 4,117,541 shares of MidWestOne's common stock; and
•: the redemption by ATBancorp of certain subordinated debentures, including any repayment fee and accrued interest, totaling approximately $9.6 million.

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Unaudited Pro Forma Condensed Combined Balance Sheet as of September 30, 2018

	*MidWestOne* Historical**			ATBancorp Historical			Pro Forma Adjustments			Notes	Pro Forma Combined

	(in thousands)
ASSETS
Cash and cash equivalents	$	53,379		$	84,857		$	(9,400	)	A	$	128,836
Equity securities, at fair value		2,797			—			—				2,797
Debt securities available for sale, at fair value		407,766			102,009			—				509,775
Debt securities held to maturity, at amortized cost		191,733			—			—				191,733
Loans held for sale		1,124			1,687			—				2,811
Loans held for investment, net of unearned income		2,377,649			1,124,893			(19,123	)	B		3,483,419
Less: allowance for loan and lease losses		31,278			12,587			(12,587	)	C		31,278

Loans held for investment, net		2,346,371			1,112,306			(6,536	)			3,452,141
Premises and equipment, net		76,497			17,451			9,600		D		103,548
Goodwill		64,654			3,253			41,950		F		109,857
Other intangible assets, net		10,378			699			12,404		G		23,481
Other real estate owned		549			3,577			(838	)	E		3,288
Other assets		112,717			39,733			(5,165	)	H		147,285

Total assets	$	3,267,965		$	1,365,572		$	42,015			$	4,675,552



LIABILITIES AND SHAREHOLDERS' EQUITY
Deposits	$	2,632,259		$	1,099,120		$	—				3,731,379
Federal funds purchased		19,056			700			—				19,756
Securities sold under agreements to repurchase		68,922			37,519			—				106,441
Other borrowed money		151,750			53,400			35,000		I		240,150
Subordinated debentures		—			20,435			(9,600	)	J		10,835
Junior subordinated notes issued to capital trusts		23,865			19,500			(4,500	)	K		38,865
Other liabilities		22,924			18,952			(94	)	L		41,782

Total liabilities		2,918,776			1,249,626			20,806				4,189,208
Commitments and contingent liabilities
Shareholders' equity:
Common stock		12,463			1,764			2,354		M		16,581
Additional paid-in capital		187,581			4,852			128,185		N		320,618
Treasury stock		(5,474	)		(585	)		585		O		(5,474	)
Retained earnings		163,709			112,204			(112,204	)	P		163,709
Accumulated other comprehensive loss		(9,090	)		(2,289	)		2,289		Q		(9,090	)

Total shareholders' equity		349,189			115,946			21,209				486,344

Total liabilities and shareholders' equity	$	3,267,965		$	1,365,572		$	42,015			$	4,675,552

See accompanying Notes to Unaudited Pro Forma Condensed Combined Financial Information.

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Unaudited Pro Forma Condensed Combined Statement of Income for the Year Ended December 31, 2017

	*MidWestOne* Historical**		ATBancorp Historical			Pro Forma Merger Adjustments			Notes	Pro Forma Combined

	(in thousands except per share amounts)
*Interest Income*
Loans	$	102,366	$	51,352		$	(3,115	)	R	$	150,603
Taxable securities		10,573		3,116			(1,257	)	S		12,432
Tax-exempt securities		6,239		439			—				6,678
Other		142		647			(341	)	T		448

Total interest income		119,320		55,554			(4,713	)			170,161
*Interest Expense*
Deposits		11,489		6,254			(753	)	U		16,990
Subordinated debentures		—		1,357			(624	)	V		733
Junior subordinated notes issued to capital trusts		949		582			225		W		1,756
Other borrowings		2,707		750			1,511		X		4,968

Total interest expense		15,145		8,943			359				24,447

*Net Interest Income*		104,175		46,611			(5,072	)			145,714
Provision for (reversal of) loan losses		17,334		(1,320	)		2,400		Y		18,414

Net interest income after provision for loan losses		86,841		47,931			(7,472	)			127,300
*Noninterest Income*
Trust, investment, and insurance fees		6,189		15,506			(11,684	)	Z		10,011
Service charges and fees on deposit accounts		5,126		1,380			(84	)	AA		6,422
Loan origination and servicing fees		3,421		3,401			(40	)	BB		6,782
Other service charges and fees		5,992		3,417			(38	)	CC		9,371
Investment securities gains, net		241		—			—				241
Other		1,401		1,459			(2,179	)	DD		681

Total noninterest income		22,370		25,163			(14,025	)			33,508
*Noninterest Expense*
Salaries and employee benefits		47,864		31,501			(11,150	)	EE		68,215
Net occupancy and equipment expense		12,305		6,696			(2,805	)	FF		16,196
Professional fees		3,962		1,658			(568	)	GG		5,052
Data processing expense		2,674		3,560			(821	)	HH		5,413
FDIC insurance expense		1,265		491			(94	)	II		1,662
Amortization of intangibles		3,125		67			2,315		JJ		5,507
Other		8,941		8,944			(3,031	)	KK		14,854

Total noninterest expense		80,136		52,917			(16,154	)			116,899

Income before income taxes		29,075		20,177			(5,343	)			43,909
Income tax provision		10,376		6,809			(2,137	)	LL		15,048

Net income before noncontrolling interest		18,699		13,368			(3,206	)			28,861
Net income attributable to noncontrolling interest		—		(788	)		788		MM		—

*Net income available to common shareholders*	$	18,699	$	12,580		$	(2,418	)		$	28,861



*Per Common Share*
Earnings basic	$	1.55	$	0.36			—			$	1.79
Earnings diluted	$	1.55	$	0.36			—			$	1.78
Weighted average number of common shares outstanding		12,038		35,070			(30,952	)	NN		16,156
Weighted average number of diluted common shares outstanding		12,063		35,070			(30,952	)	OO		16,181

See accompanying Notes to Unaudited Pro Forma Condensed Combined Financial Information.

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Unaudited Pro Forma Condensed Combined Statement of Income for the Nine Months Ended September 30, 2018

	*MidWestOne* Historical**		ATBancorp Historical			Pro Forma Merger Adjustments			Notes	Pro Forma Combined

	(in thousands except per share amounts)
*Interest Income*
Loans	$	82,141	$	37,905		$	1,800		R	$	121,846
Taxable securities		8,793		1,785			(394	)	S		10,184
Tax-exempt securities		4,452		258			—				4,710
Other		39		976			(187	)	T		828

Total interest income		95,425		40,924			1,219				137,568
*Interest Expense*
Deposits		12,170		6,475			(302	)	U		18,343
Subordinated debentures		—		574			(468	)	V		106
Junior subordinated notes issued to capital trusts		878		996			169		W		2,043
Other borrowings		3,122		1,088			1,171		X		5,381

Total interest expense		16,170		9,133			570				25,873

*Net Interest Income*		79,255		31,791			649				111,695
Provision for loan losses		4,050		804			—				4,854

Net interest income after provision for loan losses		75,205		30,987			649				106,841
*Noninterest Income*
Trust, investment, and insurance fees		4,703		13,454			(8,714	)	Z		9,443
Service charges and fees on deposit accounts		3,474		893			(26	)	AA		4,341
Loan origination and servicing fees		2,738		2,136			(14	)	BB		4,860
Other service charges and fees		4,464		2,887			(13	)	CC		7,338
Investment securities gains (losses), net		197		(1,136	)		—				(939	)
Other		1,567		24,515			(26,369	)	DD		(287	)

Total noninterest income		17,143		42,749			(35,136	)			24,756
*Noninterest Expense*
Salaries and employee benefits		37,647		22,199			(7,207	)	EE		52,639
Net occupancy and equipment expense		10,440		4,600			(1,466	)	FF		13,574
Professional fees		3,614		5,156			(1,508	)	GG		7,262
Data processing expense		2,076		2,885			(614	)	HH		4,347
FDIC insurance expense		1,104		410			(67	)	II		1,447
Amortization of intangibles		1,793		50			1,737		JJ		3,580
Other		7,026		6,206			(3,248	)	KK		9,984

Total noninterest expense		63,700		41,506			(12,373	)			92,833

Income before income taxes		28,648		32,230			(22,114	)			38,764
Income tax provision		5,921		8,790			(5,750	)	LL		8,961

Net income before noncontrolling interest		22,727		23,440			(16,364	)			29,803
Net income attributable to noncontrolling interest		—		447			(447	)	MM		—

*Net income available to common shareholders*	$	22,727	$	23,887		$	(16,811	)		$	29,803



*Per Common Share*
Earnings basic	$	1.86	$	0.67			—			$	1.82
Earnings diluted	$	1.86	$	0.67			—			$	1.82
Weighted average number of common shares outstanding		12,221		35,028			(30,910	)	NN		16,339
Weighted average number of diluted common shares outstanding		12,238		35,028			(30,910	)	OO		16,356

See accompanying Notes to Unaudited Pro Forma Condensed Combined Financial Information.

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Notes to Unaudited Pro Forma Condensed Combined Financial Information

Note 1—Basis of Presentation

The unaudited pro forma condensed combined financial information has been prepared using the acquisition method of accounting giving effect to the Merger involving MidWestOne and ATBancorp. The unaudited pro forma condensed combined financial information is presented for illustrative purposes only and is not necessarily indicative of the financial position had the Merger been consummated at September 30, 2018 or the unaudited pro forma condensed combined income statements for the year ended December 31, 2017 and the nine months ended September 30, 2018, as if the Merger had occurred as of the beginning of each period, nor is it necessarily indicative of the results of operation in future periods or the future financial position of the combined entities. The merger, which is currently expected to be completed in the first quarter of 2019, provides for the issuance of 4,117,541 shares of MidWestOne common stock and $34.8 million in cash. The value of an ATBancorp share would be $4,908.10 based upon a closing price of MidWestOne common stock of $33.31.

Under the acquisition method of accounting, the assets and liabilities of ATBancorp will be recorded at the respective fair values on the Merger date. The fair value on the Merger date represents management's best estimates based on available information and facts and circumstances in existence on the Merger date. The pro forma allocation of purchase price reflected in the unaudited pro forma condensed combined financial information is subject to adjustment and may vary from the actual purchase price allocation that will be recorded at the time the Merger is completed. Adjustments may include, but not be limited to, changes in (i) ATBancorp's balance sheet through the Effective Time; (ii) the aggregate value of Merger consideration paid if the price of MidWestOne's stock varies from the assumed $33.31 per share; (iii) total Merger related expenses if consummation and/or implementation costs vary from currently estimated amounts; and (iv) the underlying values of assets and liabilities if market conditions differ from current assumptions.

The accounting policies of both MidWestOne and ATBancorp are in the process of being reviewed in detail. Upon completion of such review, conforming adjustments or financial statement reclassification may be determined.

Note 2—Estimated Merger and Integration Costs

In connection with the Merger, the plan to integrate MidWestOne's and ATBancorp's operations is still being developed. Over the next several months, the specific details of these plans will continue to be refined. MidWestOne and ATBancorp are currently in the process of assessing the two companies' personnel, benefit plans, premises, equipment, computer systems, supply chain methodologies, and service contracts to determine where they may take advantage of redundancies or where it will be beneficial or necessary to convert to one system. Certain decisions arising from these assessments may involve involuntary termination of ATBancorp's employees, vacating ATBancorp's leased premises, changing information systems, canceling contracts between ATBancorp and certain service providers and selling or otherwise disposing of certain premises, furniture and equipment owned by ATBancorp. Additionally, as part of our formulation of the integration plan, certain actions regarding existing MidWestOne information systems, premises, equipment, benefit plans, supply chain methodologies, supplier contracts, and involuntary termination of personnel may be taken. MidWestOne expects to incur Merger-related expenses including system conversion costs, employee retention and severance agreements, communications to customers, and others. To the extent there are costs associated with these actions, the costs will be recorded based on the nature and timing of these integration actions. Most acquisition and restructuring costs are recognized separately from a business combination and generally will be expensed as incurred. We estimated the Merger related costs to be approximately $20.2 million and expect they will be incurred primarily in 2019.

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Note 3—Estimated Annual Cost Savings

Following the Merger, MidWestOne expects to realize cost savings equal to approximately 30% of ATBancorp's pre-tax noninterest expense. Management expects such cost saves to be phased-in over a two-year period, but there is no assurance that the anticipated cost savings will be realized on the anticipated time schedule or at all. These cost savings are not reflected in the presented pro forma financial information.

Note 4—Pro Forma Adjustments

The following pro forma adjustments have been reflected in the unaudited pro forma condensed combined financial information. All taxable balance sheet adjustments were calculated using a combined 26% tax rate to arrive at deferred tax asset or liability adjustments. All adjustments are based on current assumptions and valuations, which are subject to change.

Balance Sheet (dollars in thousands)

A.: Adjustments to cash and cash equivalents


To reflect cash used by ATBancorp to redeem certain subordinated debt	$	(9,600	)
To reflect excess cash proceeds from MidWestOne senior debt		200

	$	(9,400	)

B.: Adjustment to loans, net of unearned income


To reflect estimated fair value at closing date, calculated as 1.7% of the ATBancorp loan balance.		$	(19,123	)

C.: Adjustment to allowance for loan and lease losses


To remove ATBancorp allowance at Merger date as the credit risk is contemplated in the estimated fair value in adjustment B above.		$	(12,587	)

D.: Adjustment to premises and equipment


To reflect estimated fair value of ATBancorp properties at Merger date. The estimated useful life of these properties is 39 years.		$	9,600

E: Adjustment to other real estate owned


To reflect estimated fair value of ATBancorp other real estate owned at Merger date based on MidWestOne's estimate of property values given current market conditions and additional discounts necessary to liquidate these properties.		$	(838	)

F.: Adjustment to goodwill


To reverse ATBancorp goodwill on the books.	$	(3,253	)
To reflect the goodwill associated with the Merger.		45,203

	$	41,950

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G.: Adjustment to core deposit intangible ("CDI"), net


To reverse ATBancorp CDI on the books.	$	(699	)
To record the estimated fair value of acquired identifiable intangible assets, calculated as 1.5% of ATBancorp core deposits. Core deposits were identified as the demand, savings and money market accounts. The acquired core deposit intangible will be amortized over 10 years using a sum-of-the-years-digits method.		13,103

	$	12,404

H.: Adjustment to other assets

Net deferred tax liability, calculated as follows:


Adjustment to loans held for investment, net of unearned income	$	(19,123	)
Adjustment to allowance for loan and lease losses		12,587
Adjustment to CDI, net		12,404
Adjustment to premises and equipment		9,600
Adjustment to other real estate owned		(838	)
Adjustment to junior subordinated notes issued to capital trusts		4,500

Subtotal for fair value adjustments		19,130

Calculated net deferred tax liability at MidWestOne's estimated combined rate of 26%		(4,974	)

Elimination of historical other assets from the ATB Businesses.		(191	)

	$	(5,165	)

I.: Adjustment to long-term debt


To reflect proceeds from senior debt incurred by MidWestOne to fund cash portion of Merger consideration.		$	35,000

J.: Adjustment to subordinated debentures


To reflect cash redemption by ATBancorp of certain subordinated debentures.		$	(9,600	)

K.: Adjustment to junior subordinated debentures


To reflect valuation mark on junior subordinated notes issued to capital trusts.		$	(4,500	)

L.: Adjustment to other liabilities


Elimination of historical other liabilities from the ATB Businesses.		$	(94	)

M.: Adjustments to common stock


To eliminate historical ATBancorp common stock.	$	(1,764	)
To reflect the issuance of MidWestOne common stock to ATBancorp shareholders.		4,118

	$	2,354

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N.: Adjustments to additional paid-in capital


To eliminate historical ATBancorp additional paid-in capital.	$	(4,852	)
To reflect the issuance of MidWestOne common stock to ATBancorp shareholders.		133,037

	$	128,185

O.: Adjustment to treasury stock


To eliminate historical ATBancorp treasury stock		$	585

P.: Adjustment to retained earnings


To eliminate historical ATBancorp retained earnings		$	(112,204	)

Q.: Adjustment to accumulated other comprehensive income


To eliminate historical ATBancorp accumulated other comprehensive income.		$	2,289

Income Statement
(dollars and shares in thousands)

		Nine Months Ended September 30, 2018			Year Ended December 31, 2017

R.	Adjustment to loan interest income
	To reflect accretion of loan discount resulting from loan fair value pro forma adjustment based on weighted average remaining life of five years.	$	4,781		$	6,374
	To eliminate historical income from ATBancorp's United American Bank subsidiary, which was sold on May 4, 2018.		(2,981	)		(9,489	)

		$	1,800		$	(3,115	)

S.	Adjustment to taxable securities interest income
	To eliminate historical income from ATBancorp's United American Bank subsidiary, which was sold on May 4, 2018.	$	(394	)	$	(1,257	)
T.	Adjustment to other interest income
	To eliminate historical income from ATBancorp's United American Bank subsidiary, which was sold on May 4, 2018.	$	(187	)	$	(341	)
U.	Adjustment to deposit interest expense
	To eliminate historical expense from ATBancorp's United American Bank subsidiary, which was sold on May 4, 2018.	$	(302	)	$	(753	)

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		Nine Months Ended September 30, 2018			Year Ended December 31, 2017

V.	Adjustment to subordinated debentures interest expense
	Elimination of historical ATBancorp interest on subordinated debentures paid with interest calculated using an interest rate of 6.50%		(468	)		(624	)
W.	Adjustment to junior subordinated notes issued to capital trusts interest expense
	To reflect additional interest expense resulting from the amortization of the merger valuation mark on ATBancorp's junior subordinated notes.	$	169		$	225
X.	Adjustment to other debt interest expense
	To reflect additional interest expense resulting from the issuance of $35.0 million of new senior debt, at an estimated annual interest rate of 4.625%.	$	1,171		$	1,532
	To eliminate historical expense from ATBancorp's United American Bank subsidiary, which was sold on May 4, 2018.		—			(21	)

		$	1,171		$	1,511

Y.	Adjustment to reversal of loan and lease losses provision
	To eliminate historical provision expense recapture from ATBancorp's United American Bank subsidiary, which was sold on May 4, 2018.	$	—		$	2,400
Z.	Adjustment to trust, investment and insurance fees
	To eliminate estimated historical income from the ATB Businesses.	$	(8,714	)	$	(11,618	)
	To eliminate historical income from ATBancorps' United American Bank subsidiary, which was sold on May 4, 2018.		—			(66	)

		$	(8,714	)	$	(11,684	)

AA.	Adjustment to service charges and fees on deposit accounts
	To eliminate historical income from ATBancorps' United American Bank subsidiary, which was sold on May 4, 2018.	$	(26	)	$	(84	)
BB.	Adjustment to loan origination and servicing fees
	To eliminate historical income from ATBancorps' United American Bank subsidiary, which was sold on May 4, 2018.	$	(14	)	$	(40	)
CC.	Adjustment to other service charges and fees
	To eliminate historical income from ATBancorps' United American Bank subsidiary, which was sold on May 4, 2018.	$	(13	)	$	(38	)

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		Nine Months Ended September 30, 2018			Year Ended December 31, 2017

DD.	Adjustment to other noninterest income
	To eliminate historical income from ATBancorps' United American Bank subsidiary, which was sold on May 4, 2018.	$	(1	)	$	(797	)
	To eliminate estimated historical income from the ATB Businesses.		(1,037	)		(1,382	)
	To eliminate historical gain on sale from ATBancorp's sale of United American Bank subsidiary, which was sold on May 4, 2018, to be paid out as part of a special dividend prior to merger consummation.		(25,331	)		—

		$	(26,369	)	$	(2,179	)

EE.	Adjustment to salaries and employee benefits expense
	To eliminate historical expense from ATBancorps' United American Bank subsidiary, which was sold on May 4, 2018.	$	(2,525	)	$	(4,908	)
	To eliminate estimated historical expense from the ATB Businesses.		(4,682	)		(6,242	)

		$	(7,207	)	$	(11,150	)

FF.	Adjustment to occupancy expense
	To reflect additional depreciation expense resulting from premises and equipment pro forma adjustment based on estimated useful life of 39 years using the straight line method.	$	185		$	246
	To eliminate estimated historical expense from the ATB Businesses.		(806	)		(1,074	)
	To eliminate historical expense from ATBancorps' United American Bank subsidiary, which was sold on May 4, 2018.		(845	)		(1,977	)

		$	(1,466	)	$	(2,805	)

GG.	Adjustment to professional fees expense
	To eliminate historical expense from ATBancorps' United American Bank subsidiary, which was sold on May 4, 2018.	$	(1,508	)	$	(568	)
HH.	Adjustment to data processing expense
	To eliminate estimated historical expense from the ATB Businesses.		(608	)		(810	)
	To eliminate historical expense from ATBancorps' United American Bank subsidiary, which was sold on May 4, 2018.		(6	)		(11	)

		$	(614	)	$	(821	)

II.	Adjustment to FDIC insurance expense
	To eliminate historical expense from ATBancorps' United American Bank subsidiary, which was sold on May 4, 2018.	$	(67	)	$	(94	)
JJ.	Adjustment to amortization of intangibles
	To reflect amortization of acquired intangible assets based on amortization period of 10 years and using the sum-of-the-years-digits method of amortization.	$	1,787		$	2,382
	To eliminate historical CDI amortization at ATBancorp		(50	)		(67	)

		$	1,737		$	2,315

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		Nine Months Ended September 30, 2018			Year Ended December 31, 2017

KK.	Adjustment to other expense
	To eliminate estimated historical expense from the ATB Businesses.	$	(1,629	)	$	(2,172	)
	To eliminate historical expense from ATBancorps' United American Bank subsidiary, which was sold on May 4, 2018.		(1,619	)		(859	)

		$	(3,248	)	$	(3,031	)

LL.	Adjustment to income tax provision
	To reflect the income tax effect of pro forma adjustments R—JJ at MidWestOne's estimated combined tax rate of 26% for 2018 and 40% for 2017.	$	(5,750	)	$	(2,137	)
MM.	Adjustment to net income attributable to noncontrolling interests
	To eliminate historical net income attributable to noncontrolling interests stemming from ATBancorp's United American Bank subsidiary.	$	(447	)	$	788
NN.	Adjustment to weighted average number of common shares outstanding		(30,910	)		(30,952	)
	Adjustment to the year ended December 31, 2017 and the nine months ended September 30, 2018 calculated as follows:
	Removal of ATBancorp weighted average number of common shares outstanding for the year ended December 31, 2017 and the nine months ended September 30, 2018		(35,028	)		(35,070	)
	MidWestOne shares issued to ATBancorp shareholders		4,118			4,118

	Adjustment to weighted average number of common shares outstanding for the year ended December 31, 2017 and the nine months ended September 30, 2018		(30,910	)		(30,952	)

OO.	Adjustment to weighted average number of diluted common shares outstanding		(30,910	)		(30,952	)
	Adjustment to the year ended December 31, 2017 and the six nine months ended September 30, 2018 calculated as follows:
	Removal of ATBancorp weighted average number of diluted common shares outstanding for the year ended December 31, 2017 and the nine months ended September 30, 2018		(35,028	)		(35,070	)
	MidWestOne shares issued to ATBancorp shareholders		4,118			4,118

	Adjustment to weighted average number of diluted common shares outstanding for the year ended December 31, 2017 and the nine months ended September 30, 2018		(30,910	)		(30,952	)

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Note 5—Preliminary Acquisition Accounting

The table below presents the pro forma Merger consideration to be transferred by MidWestOne, the estimated fair value of assets acquired (including identifiable intangible assets) and liabilities assumed from ATBancorp, and the goodwill created by the Merger at the date indicated.

	September 30, 2018

	(in thousands)
Consideration Transferred:
MidWestOne common stock issued	$	137,155
Cash paid		34,800

Total consideration transferred	$	171,955



Assets Acquired:
Cash and cash equivalents	$	75,257
Debt securities available for sale		102,009
Loans held for sale		1,687
Loans held for investment		1,105,770
Premises and equipment		27,051
Core deposit intangible		13,103
Other real estate owned		2,739
Other assets		34,568

Total assets acquired	$	1,362,184

Liabilities Assumed:
Deposits	$	1,099,120
Borrowings		91,619
Subordinated debentures		10,835
Junior subordinated notes issued to capital trusts		15,000
Other liabilities		18,858

Total liabilities assumed	$	1,235,432

Net assets acquired	$	126,752

Goodwill	$	45,203

Comparative Historical and Unaudited Pro Forma Per Share Financial Data

The following table presents: (i) historical per share information for MidWestOne; (ii) historical per share information for ATBancorp; (iii) pro forma per share information of the combined company after giving effect to the Merger; and (iv) equivalent pro forma per share information for ATBancorp.

The combined company pro forma per share information was derived by combining information from the historical financial information presented above under "—Selected Historical Consolidated Financial Data of MidWestOne" and "—Unaudited Pro Forma Combined Financial Information." You should read this table together with the financial information discussed under those headings and the consolidated financial statements of MidWestOne incorporated by reference in this joint proxy statement/prospectus and the consolidated financial statements of ATBancorp included with this joint proxy statement/prospectus. You should not rely on the pro forma per share information as being necessarily indicative of actual results had the Merger been effective on January 1, 2017, for purposes of net income per share data, or September 30, 2018, for purposes of book value per share data.

The information appearing in the column captioned "Combined Pro Forma" in the table below was prepared assuming that 4,117,541 shares of MidWestOne common stock were issued to the

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shareholders of ATBancorp in the Merger and that the Merger was completed as of January 1, 2017, for purposes of net income per share data, and September 30, 2018, for purposes of book value per share data.

(unaudited)	MidWestOne		ATBancorp			Combined Pro Forma		Per Equivalent ATBancorp Share

Book value per share
As of December 31, 2017	$	27.85	$	3,847.11		$	29.34	$	3,449.05
As of September 30, 2018	$	28.57	$	3,310.09		$	29.77	$	3,449.05
Cash dividends
For the year ended December 31, 2017	$	0.67	$	90.00		$	0.67	$	78.76
For the nine months ended September 30, 2018	$	0.585	$	67.50	*	$	0.585	$	68.77
Basic income (loss) from continuing operations
For the year ended December 31, 2017	$	1.55	$	0.36		$	1.79	$	210.41
For the nine months ended September 30, 2018	$	1.86	$	0.67		$	1.82	$	213.94
Diluted income (loss) from continuing operations
For the year ended December 31, 2017	$	1.55	$	0.36		$	1.78	$	209.24
For the nine months ended September 30, 2018	$	1.86	$	0.67		$	1.82	$	213.94

*: Does not include a special dividend of $907.8452 per share that was distributed to ATBancorp shareholders on September 24, 2018. Such dividend represents a portion of the proceeds to ATBancorp from the sale of Heritage Commerce Corp common stock received by ATBancorp as consideration in connection with the sale, completed on May 4, 2018, of United American Bank.

Per Share Market Price Information

The following table presents (i) the closing sale price of MidWestOne common stock as reported on the NASDAQ Global Select Market on August 21, 2018, the last trading day before the date of the public announcement of the Merger Agreement, and November 26, 2018, the last practicable trading day preceding the date of this joint proxy statement/prospectus; and (ii) the equivalent pro forma value of a share of ATBancorp common stock at such dates based on the value of the consideration to be received in the Merger with respect to each share. Historical market value information regarding ATBancorp common stock is not provided because there is no active market for ATBancorp common stock.

	MidWestOne Common Stock Closing Price		Equivalent Market Value per Share of ATBancorp Common Stock

August 21, 2018	$	33.25	$	4,901.05
November 26, 2018	$	28.98	$	4,399.11

The market price of MidWestOne common stock will fluctuate between now and completion of the Merger. You should obtain a current price quotation for MidWestOne common stock.

Stock Price and Dividend Information

MidWestOne common stock trades on the NASDAQ Global Select Market under the symbol "MOFG." The following table sets forth the reported high and low sales prices of shares of

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MidWestOne common stock, and the quarterly cash dividends per share of MidWestOne common stock declared, in each case for the periods indicated.

	High		Low		Cash Dividends Declared

Year/Quarter:
2018
Fourth quarter(1)	$	34.83	$	28.22	$	0.195
Third quarter		35.20		31.92		0.195
Second quarter		34.75		31.94		0.195
First quarter		34.99		30.70		0.195
2017
Fourth quarter	$	37.94	$	30.56	$	0.17
Third quarter		35.63		31.93		0.17
Second quarter		36.72		32.92		0.165
First quarter		38.56		33.25		0.165
2016
Fourth quarter	$	39.20	$	27.93	$	0.16
Third quarter		30.74		26.50		0.16
Second quarter		30.50		25.49		0.16
First quarter		30.04		24.71		0.16

(1): Through November 26, 2018, the last practicable trading day preceding the date of this joint proxy statement/prospectus.

There is no established public trading market for shares of ATBancorp common stock. The last reported sale price of ATBancorp common stock before the date of the public announcement of the Merger Agreement on August 22, 2018, and prior to the printing of this joint proxy statement/prospectus occurred on May 16, 2018 at $3,500.00 per share. The following table sets forth for the periods indicated, dividends per share of ATBancorp common stock.

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Dividends



Year/Quarter:
2018
Fourth quarter(1)	$	—
Third quarter		22.50	(2)
Second quarter		22.50
First quarter		22.50
2017
Fourth quarter	$	35.00
Third quarter		20.00
Second quarter		20.00
First quarter		15.00
2016
Fourth quarter	$	22.00
Third quarter		10.00
Second quarter		10.00
First quarter		10.00

(1): Through November 26, 2018, the last practicable trading day preceding the date of this joint proxy statement/prospectus.
(2): ATBancorp also paid a special dividend to its shareholders of $907.8452 per share on September 24, 2018, representing a portion of the proceeds to ATBancorp from the sale of Heritage Commerce Corp common stock received as consideration in connection with the sale of United American Bank, as provided in the Merger Agreement.

As of the MidWestOne record date, the outstanding shares of MidWestOne common stock were owned by approximately 425 record owners. As of the ATBancorp record date, the outstanding shares of ATBancorp common stock were owned by approximately 18 record owners.

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RISK FACTORS

In addition to the other information contained in or incorporated by reference into this document, including the matters addressed under the caption entitled "Cautionary Statement Regarding Forward-Looking Statements," ATBancorp and MidWestOne shareholders should carefully consider the following factors in deciding whether to vote for each company's respective proposals. Please see the sections entitled "Where You Can Find More Information" in the forepart of this document and "Incorporation of Certain Documents by Reference" beginning on page 189.

Risks Related to the Merger

Because the market price of MidWestOne common stock will fluctuate, the value of the Merger consideration to be received by ATBancorp shareholders may change.

Upon completion of the Merger, each outstanding share of ATBancorp common stock, excluding Cancelled Shares and Dissenters' Shares, will be converted into the Per Share Stock Consideration and Per Share Cash Consideration pursuant to the terms of the Merger Agreement. The closing price of MidWestOne common stock on the date that the Merger is completed may vary from the closing price of MidWestOne common stock on the date MidWestOne and ATBancorp announced the Merger, on the date that this document is being mailed to MidWestOne and ATBancorp shareholders, and on the dates of the special meetings of MidWestOne and ATBancorp shareholders. Because the stock component of the Merger consideration is determined by a fixed exchange ratio, at the time of the ATBancorp special meeting, ATBancorp shareholders will not know or be able to calculate the value of the MidWestOne common stock they will receive upon completion of the Merger. Any change in the market price of MidWestOne common stock prior to completion of the Merger may affect the value of the stock component of the Merger consideration that ATBancorp shareholders will receive upon completion of the Merger. Stock price changes may result from a variety of factors, including general market and economic conditions, changes in our respective businesses, operations and prospects, and regulatory considerations, among other things. Many of these factors are beyond the control of MidWestOne and ATBancorp. ATBancorp shareholders should obtain current market quotations for shares of MidWestOne common stock before voting their shares at the ATBancorp special meeting.

MidWestOne and ATBancorp shareholders will have a reduced ownership and voting interest after the Merger and will exercise less influence over management.

MidWestOne shareholders currently have the right to vote in the election of the board of directors of MidWestOne and on other matters affecting MidWestOne. ATBancorp shareholders currently have the right to vote in the election of the board of directors of ATBancorp and on other matters affecting ATBancorp. Upon the completion of the Merger, each party's shareholders will be shareholders of MidWestOne with a percentage ownership of MidWestOne that is smaller than their current percentage ownership of MidWestOne or ATBancorp, as applicable. It is currently expected that the former shareholders of ATBancorp as a group will receive shares in the Merger constituting approximately 25.2% of the outstanding shares of the combined company's common stock immediately after the Merger. As a result, current shareholders of MidWestOne as a group will own approximately 74.8% of the outstanding shares of the combined company's common stock immediately after the Merger. Because of this, ATBancorp and MidWestOne shareholders will have less influence on the management and policies of the combined company than they now have on the management and policies of MidWestOne or ATBancorp, as applicable.

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Sales of substantial amounts of MidWestOne's common stock in the open market by former ATBancorp shareholders could depress MidWestOne's stock price.

Shares of MidWestOne common stock that are issued to shareholders of ATBancorp in the Merger will, except for those shares subject to a Lock-Up Agreement and for persons deemed to be affiliates of MidWestOne, be freely tradable without restrictions or further registration under the Securities Act. As of the MidWestOne record date, MidWestOne had approximately 12,222,145 shares of common stock entitled to vote outstanding and 445,100 shares of MidWestOne common stock were reserved for issuance under the MidWestOne Financial Group, Inc. 2017 Equity Incentive Plan. MidWestOne currently expects to issue approximately 4,117,541 shares of its common stock in connection with the Merger, of which approximately 2,514,855 shares will be subject to the Lock-Up Agreements.

If the Merger is completed and if ATBancorp's former shareholders sell substantial amounts of MidWestOne common stock in the public market following completion of the Merger, the market price of MidWestOne common stock may decrease. These sales might also make it more difficult for MidWestOne to sell equity or equity-related securities at a time and price that it otherwise would deem appropriate.

The combined company may fail to realize the anticipated benefits of the Merger.

The success of the Merger will depend on, among other things, the combined company's ability to combine the businesses of MidWestOne and ATBancorp. If the combined company is not able to successfully achieve this objective, the anticipated benefits of the Merger may not be fully realized, may not be realized at all or may take longer to realize than expected.

It is possible that the integration of MidWestOne's and ATBancorp's businesses or other factors could result in the loss or departure of key employees, the disruption of the ongoing business of MidWestOne or ATBancorp or inconsistencies in standards, controls, procedures and policies. It is also possible that customers, depositors and counterparties of MidWestOne or ATBancorp could choose to discontinue their relationships with the combined company post-Merger because they prefer doing business with an independent company or for any other reason, which would adversely affect the future performance of the combined company. These transition matters could have an adverse effect on each of MidWestOne and ATBancorp during the pre-Merger period and for an undetermined amount of time after the consummation of the Merger.

Failure of the Merger to be completed, the termination of the Merger Agreement or a significant delay in the consummation of the Merger could negatively impact MidWestOne and ATBancorp.

The Merger Agreement is subject to a number of conditions which must be fulfilled in order to complete the Merger. MidWestOne and ATBancorp have agreed to use their respective reasonable best efforts to obtain all necessary regulatory approvals for the Merger. Notwithstanding this agreement, it is a condition to MidWestOne's and ATBancorp's respective obligations to consummate the Merger that no such requisite regulatory approval shall have imposed a restriction or condition on, or requirement of, such approval that would, after the Effective Time, reasonably be expected by the board of directors of MidWestOne to materially restrict or burden, or impair in any material respect the benefits of the contemplated transactions to, or require a materially burdensome modification of, the businesses, activities, governance, legal structure, capital structure, compensation or fee arrangements of MidWestOne or its subsidiaries. Moreover, it is a condition to MidWestOne's obligation to consummate the Merger that ATBancorp successfully dispose of the ATB Businesses, which requires ATBancorp to engage in a sale process concurrently with, yet separate and apart from, the Merger.

These conditions to the consummation of the Merger may not be fulfilled and, accordingly, the Merger may not be completed. In addition, if the Merger is not completed by June 30, 2019, either

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MidWestOne or ATBancorp may choose not to proceed with the Merger, and the parties can mutually decide to terminate the Merger Agreement at any time, before or after shareholder approval.

If the Merger is delayed or not consummated, the ongoing business, financial condition and results of operations of each party may be materially adversely affected and the market price or value of each party's common stock may decline significantly, particularly to the extent that the current market price reflects a market assumption that the Merger will be consummated.

In addition, each party has incurred and will incur substantial expenses in connection with the negotiation and completion of the transactions contemplated by the Merger Agreement. If the Merger is not completed, the parties would have to recognize these expenses without realizing the expected benefits of the Merger. Any of the foregoing, or other risks arising in connection with the failure of or delay in consummating the Merger, including the diversion of management attention from pursuing other opportunities and the constraints in the Merger Agreement on the ability to make significant changes to each party's ongoing business during the pendency of the Merger, could have a material adverse effect on each party's business, financial condition and results of operations.

Additionally, MidWestOne's or ATBancorp's business may have been adversely impacted by the failure to pursue other beneficial opportunities due to the focus of management on the Merger, without realizing any of the anticipated benefits of completing the Merger, and the market price or value of MidWestOne common stock might decline, particularly to the extent that the current market price reflects a market assumption that the Merger will be completed. If the Merger Agreement is terminated and a party's board of directors seeks another merger or business combination, such party's shareholders cannot be certain that such party will be able to find a party willing to engage in a transaction on more attractive terms than the Merger.

If MidWestOne fails to successfully integrate ATBancorp into its internal control over financial reporting or if the internal control of ATBancorp over financial reporting were found to be ineffective, the integrity of MidWestOne's financial reporting could be compromised which could result in a material adverse effect on MidWestOne's reported financial results or in the determination of the effectiveness of MidWestOne's internal controls over financial reporting.

As a private company, ATBancorp has not been subject to the requirements of the Exchange Act with respect to internal control over financial reporting. The integration of ATBancorp into MidWestOne's internal control over financial reporting will require significant time and resources from management and other personnel and will increase compliance costs. If MidWestOne fails to successfully integrate the operations of ATBancorp into its internal control over financial reporting, MidWestOne's internal control over financial reporting may not be effective. Failure to achieve and maintain an effective internal control environment could have a material adverse effect on MidWestOne's ability to accurately report its financial results and the market's perception of its business and its stock price.

MidWestOne and ATBancorp will be subject to business uncertainties and contractual restrictions while the Merger is pending.

Uncertainty about the effect of the Merger on employees, customers and vendors may have an adverse effect on the business, financial condition and results of operations of ATBancorp and MidWestOne. These uncertainties may impair MidWestOne's or ATBancorp's ability to attract, retain and motivate key personnel, depositors and borrowers pending the consummation of the Merger, as such personnel, depositors and borrowers may experience uncertainty about their future roles following the consummation of the Merger. Additionally, these uncertainties could cause customers (including depositors and borrowers), suppliers, vendors and others who deal with MidWestOne or ATBancorp to seek to change existing business relationships with them or fail to extend an existing relationship with

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MidWestOne or ATBancorp. In addition, competitors may target each party's existing customers by highlighting potential uncertainties and integration difficulties that may result from the Merger.

MidWestOne and ATBancorp each have a small number of key personnel. The pursuit of the Merger and the preparation for the integration may place a burden on each company's management and internal resources. Any significant diversion of management attention away from ongoing business concerns and any difficulties encountered in the transition and integration process could have a material adverse effect on each company's business, financial condition and results of operations.

In addition, the Merger Agreement restricts ATBancorp from taking certain actions without MidWestOne's consent while the Merger is pending. These restrictions may, among other matters, prevent ATBancorp from pursuing otherwise attractive business opportunities, selling assets (except those described in the Merger Agreement), incurring indebtedness, engaging in significant capital expenditures in excess of certain limits set forth in the Merger Agreement, entering into other transactions or making other changes to its business prior to consummation of the Merger or termination of the Merger Agreement. These restrictions could have a material adverse effect on ATBancorp's business, financial condition and results of operations. Please see the section entitled "The Merger Agreement—Conduct of Business Prior to the Completion of the Merger" beginning on page 118 for a description of the restrictive covenants applicable to ATBancorp.

ATBancorp's directors and officers have interests in the Merger different from the interests of other ATBancorp shareholders.

Shares of MidWestOne common stock to be received by ATBancorp shareholders as a result of the Merger will have rights different from the shares of ATBancorp common stock.

Upon completion of the Merger, the rights of former ATBancorp shareholders will be governed by the articles of incorporation and bylaws of MidWestOne. The rights associated with ATBancorp common stock are different from the rights associated with MidWestOne common stock. Please see the section entitled "Comparison of Shareholders' Rights" beginning on page 134 for a summary of the different rights associated with MidWestOne common stock.

The Merger Agreement contains provisions that may discourage other companies from trying to acquire ATBancorp.

The Merger Agreement contains provisions that may discourage a third party from submitting a business combination proposal to ATBancorp that might result in greater value to ATBancorp's shareholders than that provided pursuant to the Merger. These provisions include a general prohibition on ATBancorp from soliciting or providing information to any third party regarding any acquisition proposal or offers for competing transactions, subject to certain exceptions set forth in the Merger Agreement. Further, the Voting Agreement requires that the shareholders party thereto vote all of their shares of ATBancorp common stock in favor of the Merger and against alternative acquisition proposals. The shareholders that are party to the voting agreements described in this paragraph

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beneficially own in the aggregate 62.41% of the outstanding shares of ATBancorp common stock as of the ATBancorp record date. For further information, please see "The Merger Agreement—Voting Agreement" beginning on page 127. Each party also has an obligation to submit its respective proposals to a vote by its shareholders.

Combining the two companies may be more difficult, costly or time consuming than expected and the anticipated benefits and cost savings of the Merger may not be realized.

MidWestOne and ATBancorp have operated and, until the completion of the Merger, will continue to operate, independently. The success of the Merger, including anticipated benefits and cost savings, will depend, in part, on the successful combination of the businesses of MidWestOne and ATBancorp. To realize these anticipated benefits and cost savings, after the completion of the Merger, MidWestOne expects to integrate ATBancorp's business into its own. It is possible that the integration process could result in the loss of key employees, the disruption of each company's ongoing businesses or inconsistencies in standards, controls, procedures and policies that adversely affect the combined company's ability to maintain relationships with clients, customers, depositors and employees or to achieve the anticipated benefits and cost savings of the Merger. The loss of key employees could have an adverse effect on the companies' financial results and the value of their common stock. If MidWestOne experiences difficulties with the integration process, the anticipated benefits of the Merger may not be realized fully or at all, or may take longer to realize than expected.

The combined company expects to incur substantial expenses related to the Merger.

The combined company expects to incur substantial expenses in connection with consummation of the Merger and combining the business, operations, networks, systems, technologies, policies and procedures of the two companies. Although MidWestOne and ATBancorp have assumed that each party would incur a certain level of transaction and combination expenses, there are a number of factors beyond their control that could affect the total amount or the timing of their transaction combination expenses. Many of the expenses that will be incurred, by their nature, are difficult to estimate accurately at the present time. Due to these factors, the transaction and combination expenses associated with the Merger could, particularly in the near term, exceed the savings that the combined company expects to achieve from the elimination of duplicative expenses and the realization of economies of scale and cost savings related to the combination of the businesses following the consummation of the Merger. As a result of these expenses, both MidWestOne and ATBancorp could take charges against their earnings before and after the completion of the Merger. Such charges taken in connection with the Merger could be significant, although the aggregate amount and timing of any such charges are uncertain at present.

The unaudited pro forma combined condensed consolidated financial information included in this document is illustrative only and the actual financial condition and results of operations after the Merger may differ materially.

The unaudited pro forma combined condensed consolidated financial information in this document is presented for illustrative purposes only and is not necessarily indicative of what MidWestOne's actual financial condition or results of operations would have been had the Merger been completed on the dates indicated. The pro forma combined condensed consolidated financial information reflects adjustments, which are based upon preliminary estimates, to record the ATBancorp identifiable tangible and intangible assets acquired and liabilities assumed at fair value and the resulting goodwill recognized. Accordingly, the final acquisition accounting adjustments may differ materially from the pro forma adjustments reflected in this document. For more information, please see the section entitled "Certain Financial Information Regarding MidWestOne and ATBancorp—Unaudited Pro Forma Condensed Combined Financial Information" beginning on page 28.

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The opinion of ATBancorp's financial advisor delivered to ATBancorp's board of directors, and the opinion of MidWestOne's financial advisor delivered to MidWestOne's board of directors, in each case prior to execution of the Merger Agreement, will not reflect any changes in circumstances that may have occurred since the respective dates of the opinions.

The fairness opinion of ATBancorp's financial advisor was delivered to ATBancorp's board of directors on August 20, 2018, and the fairness opinion of MidWestOne's financial advisor was delivered to MidWestOne's board of directors on August 21, 2018. Changes in the operations and prospects of ATBancorp or MidWestOne, general market and economic conditions and other factors which may be beyond the control of ATBancorp and MidWestOne may have altered the value of ATBancorp or MidWestOne or the prices of shares of ATBancorp common stock or MidWestOne common stock as of the date of this joint proxy statement/prospectus, or may alter such values and prices by the time the Merger is completed. The opinions from ATBancorp's and MidWestOne's respective financial advisors do and will not speak as of the date of this joint proxy statement/prospectus or the time the Merger is completed or any other date other than the respective dates of those opinions. For a description of the opinions of ATBancorp's and MidWestOne's respective financial advisors, please refer to the sections entitled "The Merger—Opinion of ATBancorp's Financial Advisor" beginning on page 86 and "The Merger—Opinion of MidWestOne's Financial Advisor" beginning on page 70.

ATBancorp shareholders will pay U.S. federal income tax on the Per Share Cash Consideration and cash received in connection with any fractional shares.

The Per Share Cash Consideration and cash received in connection with any fractional shares that ATBancorp shareholders receive in connection with the Merger will be taxable for U.S. federal income tax purposes. Each ATBancorp shareholder should consult its own tax advisor as to the tax consequences of the Merger in its particular circumstances, including the applicability and effect of the alternative minimum tax and any state, local, foreign or other tax laws and of changes in those laws. This joint proxy statement/prospectus does not address any tax consequences arising under the laws of any state, local or foreign jurisdiction, or under any U.S. federal laws other than those pertaining to the income tax nor does it address any tax consequences arising under the unearned income Medicare contribution tax pursuant to the Health Care and Education Reconciliation Act of 2010. See "Material U.S. Federal Income Tax Consequences of the Merger" beginning on page 129.

The amount of consideration to be received by shareholders exercising their appraisal rights is uncertain.

ATBancorp shareholders will have the right to demand appraisal of their shares of ATBancorp common stock and obtain payment in cash for the fair value of their shares, but only if they perfect their appraisal rights and comply with the applicable provisions of Iowa law. ATBancorp shareholders may or may not be entitled to receive more than the amount provided for in the Merger Agreement for their shares of ATBancorp common stock if they elect to exercise their appraisal rights with respect to the proposed Merger, depending on the appraisal of the fair value of the ATBancorp common stock. For this reason, the amount of cash that an ATBancorp shareholder might be entitled to receive should it elect to exercise its appraisal rights may be more or less than the value of the Merger consideration to be paid pursuant to the Merger Agreement. In addition, it is a condition in the Merger Agreement that the holders of not more than 7.5% of the outstanding shares of ATBancorp common stock shall have exercised their statutory dissenters' rights under Iowa law. The number of shares of ATBancorp shareholders that will exercise appraisal rights is not known and therefore there is no assurance of this closing condition being satisfied.

MidWestOne will be required to issue debt in order to finance the Per Share Cash Consideration.

As part of the Merger consideration, MidWestOne will pay each ATBancorp shareholder the Per Share Cash Consideration for each share of ATBancorp common stock owned by the shareholder. In

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order to pay the Per Share Cash Consideration, MidWestOne will be required to issue equity or debt, the proceeds from which would be used to pay the Per Share Cash Consideration. While MidWestOne currently expects to issue senior debt in order to finance the payment of the aggregate Per Share Cash Consideration, it is possible that MidWestOne will not be able to complete such an offering, or will not be able to complete such an offering on terms favorable to MidWestOne. Further, an issuance of senior debt could require MidWestOne to pledge its ownership interest in MidWestOne Bank, which could limit MidWestOne's ability to secure additional financing in the future.

Risks Related to the Ownership of MidWestOne Common Stock

An investment in MidWestOne common stock is not an insured deposit.

MidWestOne's common stock is not a bank deposit and, therefore, is not insured against loss by the FDIC, any other deposit insurance fund or by any other public or private entity. Investment in MidWestOne common stock is inherently risky for the reasons described in this "Risk Factors" section and elsewhere in this report and is subject to the same market forces that affect the price of common stock in any company. As a result, an ATBancorp shareholder who acquires MidWestOne common stock in the Merger could lose some or all of the shareholder's investment.

The trading volume in MidWestOne common stock is less than that of other larger financial services companies.

Although MidWestOne's common stock is listed for trading on the NASDAQ Global Select Market, its trading volume is generally less than that of other, larger financial services companies, and investors are not assured that a liquid market will exist at any given time for MidWestOne common stock. A public trading market having the desired characteristics of depth, liquidity and orderliness depends on the presence in the marketplace at any given time of willing buyers and sellers of MidWestOne common stock. This presence depends on the individual decisions of investors and general economic and market conditions over which MidWestOne has no control. Given the lower trading volume of MidWestOne's common stock, significant sales of MidWestOne common stock, or the expectation of these sales, could cause MidWestOne's stock price to fall.

The holders of MidWestOne's junior subordinated debentures have rights that are senior to those of MidWestOne's shareholders.

MidWestOne currently has outstanding an aggregate of $23.8 million in junior subordinated debentures in connection with trust preferred securities issuances by MidWestOne's statutory trust subsidiaries and will assume pursuant to the Merger an additional $19.5 million in junior subordinated debentures in connection with trust preferred securities issuances by ATBancorp's statutory trust subsidiaries and $10.8 million of ATBancorp's subordinated debentures. MidWestOne conditionally guarantees payments of the principal and interest on the trust preferred securities. The junior subordinated debentures and the subordinated debentures are senior to MidWestOne's shares of common stock. As a result, MidWestOne must make payments on the junior subordinated debentures (and the related trust preferred securities) and the subordinated debentures before any dividends can be paid on MidWestOne common stock and, in the event of MidWestOne's bankruptcy, dissolution or liquidation, the holders of the debentures must be satisfied before any distributions can be made to the holders of MidWestOne common stock. Additionally, MidWestOne has the right to defer periodic distributions on the junior subordinated debentures (and the related trust preferred securities) for up to 20 consecutive quarters, during which time MidWestOne would be prohibited from paying dividends on its common stock. MidWestOne's ability to pay the future distributions depends upon the earnings of MidWestOne Bank and the dividends from MidWestOne Bank to MidWestOne, which may be inadequate to service the obligations.

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CAUTIONARY STATEMENT REGARDING FORWARD-LOOKING STATEMENTS

Certain statements contained in this document that are not statements of historical fact constitute forward-looking statements within the meaning of the Private Securities Litigation Reform Act of 1995, notwithstanding that such statements are not specifically identified as such. Examples of forward-looking statements include, but are not limited to: (i) projections of revenues, expenses, income or loss, earnings or loss per share, the payment or nonpayment of dividends, capital structure and other financial items; (ii) statements of plans, objectives and expectations of MidWestOne, ATBancorp or their respective management or boards of directors, including those relating to products or services; (iii) statements of future economic performance; and (iv) statements of assumptions underlying such statements. Words such as "believes," "anticipates," "expects," "intends," "targeted," "projected," "continue," "remain," "will," "should," "could," "may" and other similar expressions are intended to identify forward-looking statements but are not the exclusive means of identifying such statements and the lack of such an identifying word does not necessarily indicate the absence of a forward-looking statement.

Forward-looking statements are based on assumptions and involve risks and uncertainties, many of which are beyond MidWestOne's and ATBancorp's control, which may cause actual results to differ materially from those discussed in such statements. Factors that could cause actual results to differ from those discussed in the forward-looking statements include, but are not limited to:

•: the ability to obtain regulatory approvals and meet other closing conditions to the Merger, including approval by MidWestOne's shareholders and ATBancorp's shareholders, on the expected terms and schedule;
•: delays in closing the Merger;
•: difficulties and delays in integrating the MidWestOne and ATBancorp businesses, fully realizing cost savings and other benefits or that or such integration may be more difficult, time consuming or costly than expected;
•: expected revenue synergies and cost savings from the Merger may not be fully realized or realized within the expected time frame;