Company: OSBC
Filing Date: 2025-06-30
Form Type: 11-K
Source: 0000357173-25-000063
Chunk: 0

Company: OLD SECOND BANCORP INC
Filing Date: 2025-06-30
Form: 11-K
Chunk 0
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I

#### ​

### UNITED STATES

### SECURITIES AND EXCHANGE COMMISSION
<div align='center'>Washington, D.C. 20549</div>

#### Form 11-K

### ANNUAL REPORT
#### ​

<div align='center'>**Pursuant to Section 15 (d) of the Securities Exchange Act of 1934

xANNUAL REPORT PURSUANT TO SECTION 15(d) OF

THE SECURITIES EXCHANGE ACT OF 1934

For the fiscal year ended December 31, 2024

OR

oTRANSITION REPORT PURSUANT TO SECTION 15(d) OF

THE SECURITIES EXCHANGE ACT OF 1934

For the transition period fromto ____________

Commission file number 0-10537

A. Full title of the plan and the address of the plan if different from that of the issuer named below:

Old Second Bancorp, Inc. Employees’ 401(k) Savings Plan and Trust

B. Name of the issuer of the securities held pursuant to the plan and the address of its principal executive office:

37 South River Street, Aurora, Illinois 60507

(Address of principal executive offices, including zip)

(630) 892-0202

(Registrant’s telephone number, including Area Code)**</div>

Financial Statements and Supplemental Schedule

Old Second Bancorp, Inc. Employees’

401(k) Savings Plan and Trust

As of December 31, 2024 and 2023, and the year ended December 31, 2024 with Report of Independent Registered Public Accounting Firm

Employer Identification #36-3143493

Plan #003

<div align='center'>2</div>

Old Second Bancorp, Inc. Employees’ 401(k) Savings Plan and Trust Financial Statements and Supplemental Schedule As of December 31, 2024 and 2023, and the year ended December 31, 2024 Contents

| ​                                                              |    ​ |
| Report of Independent Registered Public Accounting Firm        |    4 |
| ​                                                              |    ​ |
| Financial Statements                                           |    ​ |
| Statements of Net Assets Available for Benefits                |    5 |
| Statement of Changes in Net Assets Available for Benefits      |    6 |
| Notes to Financial Statements                                  | 7-11 |
| ​                                                              |    ​ |
| Supplemental Schedule​                                         |    ​ |
| Schedule H, Line 4i – Schedule of Assets (H