Company: OSBC
Filing Date: 2025-05-06
Form Type: S-4/A
Source: 0001104659-25-045103
Chunk: 4

Company: OLD SECOND BANCORP INC
Filing Date: 2025-05-06
Form: S-4/A
Chunk 4
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 are incorporated into this proxy statement/prospectus by reference. Your vote is very important, regardless of the number of shares you own. Whether or not you expect to attend the Bancorp Financial special meeting, the details of which are described in the accompanying proxy statement/prospectus, please promptly submit your proxy by the Internet or by completing, signing, dating and returning your signed proxy card(s) in the enclosed prepaid return envelope so that your shares may be represented at the special meeting. If Bancorp Financial stockholders have any questions or require assistance in voting their shares of Bancorp Financial common stock, they should call our Corporate Secretary, Mr. Michael Spaniak, at (630) 413-9580. Sincerely, Darin Campbell
Chairman, President and Chief Executive Officer
Bancorp Financial, Inc. Neither the U.S. Securities and Exchange Commission nor any state securities commission has approved or disapproved of the merger, the issuance of shares of Old Second common stock in connection with the merger or the other transactions described in this proxy statement/prospectus, or passed upon the adequacy or accuracy of the disclosure in this proxy statement/prospectus. 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 or savings association and are not insured by the Federal Deposit Insurance Corporation or any other governmental agency. This proxy statement/prospectus is dated [•], 2025, and is first being mailed to Bancorp Financial stockholders on or about [•], 2025.

TABLE OF CONTENTS

#### 1515 West 22nd Street, Suite 100W

#### Oak Brook, Illinois 60523
<div align='center'>**NOTICE OF SPECIAL MEETING OF STOCKHOLDERS TO BE HELD JUNE 18, 2025**</div>

To the Stockholders of Bancorp Financial, Inc., which we refer to as “Bancorp Financial”:

We are pleased to invite you to attend the special meeting of stockholders of Bancorp Financial, which we refer to as the “Bancorp Financial special meeting,” to be held on June 18, 2025 at 9:00 A.M. Central Time. The Bancorp Financial special meeting will be held virtually only, via the internet — there will be no physical location for the special meeting.

At the Bancorp Financial special meeting you will be asked to vote on the following proposals:

1.

Merger