Document and Entity Information - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2021
Mar. 07, 2022
Jun. 30, 2021
Document and Entity Information      
Document Type 10-K    
Document Annual Report true    
Document Transition Report false    
Document Period End Date Dec. 31, 2021    
Entity File Number 000-10537    
Entity Registrant Name OLD SECOND BANCORP INC    
Entity Incorporation, State or Country Code DE    
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    
Title of 12(b) Security Common Stock    
Trading Symbol OSBC    
Security Exchange Name NASDAQ    
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
ICFR Auditor Attestation Flag true    
Entity Shell Company false    
Entity Public Float     $ 341.6
Entity Common Stock, Shares Outstanding   44,461,045  
Current Fiscal Year End Date --12-31    
Document Fiscal Year Focus 2021    
Document Fiscal Period Focus FY    
Entity Central Index Key 0000357173    
Amendment Flag false    
Auditor Name Plante & Moran PLLC    
Auditor Firm ID 166    
Auditor Location Chicago, Illinois