| Document and Entity Information | Oct. 20, 2021 |
| Document and Entity Information [Abstract] | |
| Document Type | 8-K |
| Document Period End Date | Oct. 20, 2021 |
| Entity Registrant Name | OLD SECOND BANCORP INC |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 000-10537 |
| Entity Tax Identification Number | 36-3143493 |
| Entity Address, Address Line One | 37 South River Street |
| Entity Address, City or Town | Aurora |
| Entity Address, State or Province | IL |
| Entity Address, Postal Zip Code | 60507 |
| City Area Code | 630 |
| Local Phone Number | 892-0202 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock |
| Trading Symbol | OSBC |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0000357173 |
| Amendment Flag | false |
