v3.26.1
Borrowings - Additional information (Details)
$ in Thousands
3 Months Ended 6 Months Ended 12 Months Ended
Apr. 15, 2026
USD ($)
Jun. 30, 2026
USD ($)
item
Jun. 30, 2021
USD ($)
Jun. 30, 2026
USD ($)
item
Dec. 31, 2025
USD ($)
Jun. 30, 2025
USD ($)
Borrowings            
Average amount outstanding   $ 21,300     $ 35,400  
Average rate paid   0.79%     0.71%  
Securities sold under repurchase agreements   $ 23,241   $ 23,241 $ 23,769  
Threshold percentage of stockholders' equity       10.00%    
Borrowings at FHLBC as percentage of total assets   35.00%   35.00%    
Notes payable and other borrowings   $ 14,900   $ 14,900 14,800  
FHLBC stock   19,400   19,400 11,300  
Redemption of junior subordinated debt       30,000    
Outstanding, net of deferred issuance costs   29,800   29,800 59,600  
Undrawn line of credit   30,000   30,000    
Asset Pledged as Collateral with Right | Securities sold under repurchase agreements            
Borrowings            
Carrying amount of securities secured   73,400   73,400 74,000  
Maximum            
Borrowings            
Securities sold under repurchase agreements   23,200   23,200   $ 47,300
Securities sold under repurchase agreements            
Borrowings            
Carrying amount of securities secured   $ 23,200   $ 23,200 23,800  
Number of customers having secured balances exceeding specified percentage of stockholders equity | item   0   0    
Federal Home Loan Bank Advances            
Borrowings            
FHLBC advance amount   $ 375,000   $ 375,000 $ 215,000  
FHLBC stock   19,400   19,400    
Principal balance of loans collateralized   1,430,000   1,430,000    
Combined collateral value   938,600   938,600    
Amount available for excess collateral   $ 547,300   $ 547,300    
Subordinated Notes Due 2031            
Borrowings            
Face amount $ 60,000   $ 60,000      
Interest rate (as a percent) 3.50% 6.41% 3.50% 6.41%    
Investment, Variable Interest Rate, Type [Extensible Enumeration] osbc:ThreeMonthsSecuredOvernightFinancingRateMember   osbc:ThreeMonthsSecuredOvernightFinancingRateMember      
Basis points added to reference rate (as a percent) 2.73%   2.73%      
Redemption of junior subordinated debt $ 30,000