Document and Entity Information
Jul. 07, 2021
Document and Entity Information [Abstract]  
Document Type 8-K
Document Period End Date Jul. 07, 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