v3.26.1
Derivative Financial Instruments - Gains and Losses Recognized on Derivative Instruments Not Designated as Hedging Instruments (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2026
Jun. 30, 2025
Jun. 30, 2026
Jun. 30, 2025
Derivatives designated as fair value hedging instruments:        
Derivative Instruments, Gain (Loss) [Line Items]        
Amount of gain (loss) recognized in interest income on available for sale securities $ (606) $ 700 $ (878) $ 700
Derivatives designated as fair value hedging instruments: | Interest rate swaps        
Derivative Instruments, Gain (Loss) [Line Items]        
Amount of gain (loss) recognized in interest income on available for sale securities 606 (700) 878 (700)
Income Statement Location [Axis]: us-gaap:NoninterestIncomeOtherOperatingIncome        
Derivative Instruments, Gain (Loss) [Line Items]        
Amount of gain (loss) recognized on derivatives not designated as hedging instruments 18 (35) 29 (99)
Income Statement Location [Axis]: obnk:MortgageBankingRevenue        
Derivative Instruments, Gain (Loss) [Line Items]        
Amount of gain (loss) recognized on derivatives not designated as hedging instruments $ 0 $ (300) $ 0 $ (185)