Company: OSBC
Filing Date: 2025-05-06
Form Type: S-4/A
Source: 0001104659-25-045103
Chunk: 0

Company: OLD SECOND BANCORP INC
Filing Date: 2025-05-06
Form: S-4/A
Chunk 0
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TABLE OF CONTENTS

As filed with the U.S. Securities and Exchange Commission on May 6, 2025 File No. 333-286687 UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549 Amendment No. 1
to FORM S-4 REGISTRATION STATEMENT UNDER
THE SECURITIES ACT OF 1933 OLD SECOND BANCORP, INC.
(Exact Name of Registrant as Specified in its Charter)

| ​ | Delaware                                                       
 (State or Other Jurisdictionof Incorporation or Organization)​ | ​ | ​ | 6022                                                     
 (Primary Standard IndustrialClassification Code Number)​ | ​ | ​ | 36-3143493                          
 (IRS EmployerIdentification Number) | ​ |

37 South River Street
Aurora, Illinois 60507
(630) 892-0202 (Address, Including Zip Code, and Telephone Number, Including Area Code, of Registrant’s Principal Executive Offices) James L. Eccher
Chief Executive Officer and President
Old Second Bancorp, Inc.
37 South River Street
Aurora, Illinois 60507
(630) 892-0202 (Name, Address, Including Zip Code, and Telephone Number, Including Area Code, of Agent for Service) Copies to:

| ​ | Brennan RyanBrittany M. McIntoshNelson Mullins Riley & Scarborough LLPAtlantic Station201 17thStreet NW, Suite 1700Atlanta, Georgia 30363(404) 322-6000 | ​ | ​ | Daniel C. McKay, IIJennifer Durham KingVedder Price P.C.222 North LaSalle Street, Suite 2600Chicago, Illinois 60601(312) 609-7500 | ​ |

Approximate date of commencement of proposed sale of the securities to the public: As soon as practicable after this Registration Statement becomes effective and upon completion of the merger described in the enclosed proxy statement/prospectus. If the securities being registered on this form are being offered in connection with the formation of a holding company and there is compliance with General Instruction G, check the following box: ☐ If this form is filed to register additional securities for an offering pursuant to Rule 462(b) under the Securities Act, check the following box and list the Securities Act registration statement number of the earlier effective registration statement for the same offering. ☐ If this form is a post-effective amendment filed pursuant to Rule 462(d) under the Securities