Company: OSBC
Filing Date: 2025-04-23
Form Type: S-4
Source: 0001104659-25-037832
Chunk: 7

Company: OLD SECOND BANCORP INC
Filing Date: 2025-04-23
Form: S-4
Chunk 7
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 number or by using the Internet as described in the instructions included with the accompanying proxy card. If your shares are held in the name of a bank, broker or other nominee, please follow the instructions on the voting instruction card furnished by such bank, broker or other nominee.

If you have any questions or need assistance with voting, please contact our Corporate Secretary, Mr. Michael Spaniak, Corporate Secretary by calling (630) 413-9580.

BY ORDER OF THE BOARD OF DIRECTORS,

Darin Campbell 
 Chairman, President and Chief Executive Officer

[•], 2025 
 Oak Brook, Illinois

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### WHERE YOU CAN FIND MORE INFORMATION
This proxy statement/prospectus incorporates important business and financial information about Old Second from documents filed with the U.S. Securities and Exchange Commission, which we refer to as the “SEC,” that are not included in or delivered with this proxy statement/prospectus. You can obtain any of the documents filed with or furnished to the SEC by Old Second at no cost from the SEC’s website at http://www.sec.gov.

The reports and other information filed by Old Second with the SEC are also available at Old Second’s website at www.oldsecond.com under the “Investor Relations” tab. Except as specifically incorporated by reference into this proxy statement/prospectus, information on Old Second’s website is not part of this proxy statement/prospectus.

You may also request copies of these documents, including documents incorporated by reference in this proxy statement/prospectus, at no cost by contacting Old Second at the following address:

<div align='center'>Old Second Bancorp, Inc. 
 37 South River Street 
 Aurora, Illinois 60507 
 Attention: Shirley Cantrell 
 Telephone: (630) 906-2303</div>

You will not be charged for any of these documents that you request. To obtain timely delivery of these documents, you must request them no later than five business days before the date of the Bancorp Financial special meeting. This means that holders of Bancorp Financial common stock requesting documents must do so by [ • ], 2025 in order to receive them before the Bancorp Financial special meeting.

Please see “Incorporation of Certain Documents by Reference” for more further information.

<div align='center'>i</div>

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#### ABOUT THIS PROXY STATEMENT/PROSPECTUS
This proxy statement/prospectus, which forms part of a registration statement on Form S-4 filed with the SEC by