v3.26.3
Borrowings - Additional information (Details)
$ in Thousands
3 Months Ended 9 Months Ended 12 Months Ended
Sep. 14, 2026
USD ($)
Apr. 15, 2026
USD ($)
Sep. 30, 2026
USD ($)
item
Jun. 30, 2021
USD ($)
Sep. 30, 2026
USD ($)
item
Dec. 31, 2025
USD ($)
Jun. 30, 2026
USD ($)
Jun. 30, 2025
USD ($)
Borrowings                
Average amount outstanding     $ 24,100     $ 33,400    
Average rate paid     0.73%     0.71%    
Securities sold under repurchase agreements     $ 29,908   $ 29,908 $ 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     $ 9,900   $ 9,900 14,800    
Balance of borrowings paid off $ 5,000              
Unamortized purchase accounting adjustments recognized as interest expense $ 31,000              
FHLBC stock     20,100   20,100 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      
Repayment of term note         5,000      
Asset Pledged as Collateral with Right | Securities sold under repurchase agreements                
Borrowings                
Carrying amount of securities secured     54,000   54,000 74,000    
Maximum                
Borrowings                
Securities sold under repurchase agreements             $ 29,900 $ 34,800
Securities sold under repurchase agreements                
Borrowings                
Carrying amount of securities secured     $ 29,900   $ 29,900 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     $ 395,000   $ 395,000 $ 215,000    
FHLBC stock     20,100   20,100      
Principal balance of loans collateralized     1,430,000   1,430,000      
Combined collateral value     950,900   950,900      
Amount available for excess collateral     $ 544,600   $ 544,600      
Subordinated Notes Due 2031                
Borrowings                
Face amount   $ 60,000   $ 60,000        
Interest rate (as a percent)   3.50% 6.48% 3.50% 6.48%      
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