Company: OSBC
Filing Date: 2025-05-06
Form Type: S-4/A
Source: 0001104659-25-045103
Chunk: 176

Company: OLD SECOND BANCORP INC
Filing Date: 2025-05-06
Form: S-4/A
Chunk 176
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ive Proxy Statement on Schedule 14A | ​ | ​ | Filed April 18, 2025                                                                                                                                                                                   | ​ |

Old Second also incorporates by reference the description of Old Second capital stock contained as Exhibit 4.5 to Old Second’s Annual Report on Form 10-K for the year ended December 31, 2024, and any amendment or report filed for the purpose of updating such description. In addition, Old Second also incorporates by reference additional documents that it may file with the SEC under Section 13(a), 13(c), 14 or 15(d) of the Exchange Act after the date of this proxy statement/ prospectus and prior to the date of the Bancorp Financial special meeting, provided, however, that Old Second is not incorporating by reference any information furnished (but not filed), except as otherwise specified herein. Old Second files annual, quarterly and current reports, proxy statements and other business and financial information with the SEC. You may obtain the information incorporated by reference and any other materials Old Second files with the SEC without charge by following the instructions in the section of this proxy statement/prospectus entitled “Where You Can Find More Information” in the forepart of this proxy statement/prospectus.

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TABLE OF CONTENTS

#### Bancorp Financial, Inc.and Subsidiary
<div align='center'>Consolidated Financial Report 
 December 31, 2024

F-1</div>

TABLE OF CONTENTS

Contents

| ​ | Independent Auditor’s Report                    | ​ | ​ | F-3– F-4   | ​ |
| ​ | Consolidated Financial Statements               | ​ | ​ | ​          | ​ |
| ​ | Consolidated Balance Sheets                     | ​ | ​ | F-5        | ​ |
| ​ | Consolidated Statements of Income               | ​ | ​ | F-6        | ​ |
| ​ | Consolidated Statements of Comprehensive Income | ​ | ​ | F-7        | ​ |
| ​ | Consolidated Statements of Stockholders’ Equity | ​ | ​ | F-8        | ​ |
| ​ | Consolidated Statements of Cash Flows           | ​ | ​ | F-9        | ​ |
| ​ | Notes to Consolidated Financial Statements      | ​ | ​ | F-10– F-36 | ​ |

F-2

TABLE OF CONTENTS

#### Independent Auditor’s Report
Board of Directors 
 Bancorp Financial, Inc. and Subsidiary

### Report on