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Securities - Contractural Maturities (Details) (USD $)
In Thousands, unless otherwise specified
 3 Months Ended
Mar. 31, 2015
Dec. 31, 2014
Securities Available-for-Sale, Amortized Cost    
Due in one year or less $ 20,410us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCost  
Due after one year through five years 9,048us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCost  
Due after five years through ten years 35,588us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsAmortizedCost  
Due after ten years 5,140us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsAmortizedCost  
Debt securities excluding securities not due at a single maturity date 70,186osbc_AvailableForSaleDebtMaturitiesBeforeSecuritiesWithoutSingleMaturityDateAmortizedCostBasis  
Total 404,429us-gaap_AvailableForSaleSecuritiesAmortizedCost 391,694us-gaap_AvailableForSaleSecuritiesAmortizedCost
Securities Available-for-Sale, Weighted Average Yield    
Due in one year or less (as a percent) 1.56%osbc_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearAmortizedCostWeightedAverageYield  
Due after one year through five years (as a percent) 2.64%osbc_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsAmortizedCostWeightedAverageYield  
Due after five years through ten years (as a percent) 2.16%osbc_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsAmortizedCostWeightedAverageYield  
Due after ten years (as a percent) 3.27%osbc_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsAmortizedCostWeightedAverageYield  
Debt securities (as a percent) 2.13%osbc_AvailableForSaleDebtMaturitiesBeforeSecuritiesWithoutSingleMaturityDateWeightedAverageYield  
Total (as a percent) 1.84%osbc_AvailableForSaleSecuritiesWeightedAverageYield  
Securities Available-for-Sale, Fair Value    
Due in one year or less 20,454us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue  
Due after one year through five years 9,199us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterOneThroughFiveYearsFairValue  
Due after five years through ten years 35,221us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterFiveThroughTenYearsFairValue  
Due after ten years 5,012us-gaap_AvailableForSaleSecuritiesDebtMaturitiesAfterTenYearsFairValue  
Debt securities 69,886osbc_AvailableForSaleDebtMaturitiesBeforeSecuritiesWithoutSingleMaturityDateFairValue  
Fair Value 399,252us-gaap_AvailableForSaleSecurities 385,486us-gaap_AvailableForSaleSecurities
Securities Held-to-Maturity, Amortized Cost    
Amortized Cost 257,332us-gaap_HeldToMaturitySecurities 259,670us-gaap_HeldToMaturitySecurities
Securities Held-to-Maturity, Weighted Average Yield    
Weighted Average Yield (as a percent) 3.03%osbc_HeldToMaturitySecuritiesWeightedAverageYield  
Securities Held-to-Maturity, Fair Value    
Fair Value 265,095us-gaap_HeldToMaturitySecuritiesFairValue 263,266us-gaap_HeldToMaturitySecuritiesFairValue
Collateralized mortgage obligations    
Securities Available-for-Sale, Amortized Cost    
Securities not due at a single maturity date 69,205us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember		  
Total 69,205us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember		 65,728us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
Securities Available-for-Sale, Weighted Average Yield    
Securities not due at a single maturity date, Weighted Average Yield (as a percent) 1.95%osbc_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateWeightedAverageYield
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember		  
Securities Available-for-Sale, Fair Value    
Securities not due at a single maturity date 68,093us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember		  
Fair Value 68,093us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember		 63,627us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
Securities Held-to-Maturity, Amortized Cost    
Amortized Cost 220,197us-gaap_HeldToMaturitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember		 222,545us-gaap_HeldToMaturitySecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
Securities Held-to-Maturity, Fair Value    
Fair Value 225,419us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember		 224,111us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedMortgageObligationsMember
Asset-backed securities    
Securities Available-for-Sale, Amortized Cost    
Securities not due at a single maturity date 170,817us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember		  
Total 170,817us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember		 175,565us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
Securities Available-for-Sale, Weighted Average Yield    
Securities not due at a single maturity date, Weighted Average Yield (as a percent) 1.11%osbc_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateWeightedAverageYield
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember		  
Securities Available-for-Sale, Fair Value    
Securities not due at a single maturity date 168,256us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember		  
Fair Value 168,256us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember		 173,496us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_AssetBackedSecuritiesMember
Collateralized loan obligations    
Securities Available-for-Sale, Amortized Cost    
Securities not due at a single maturity date 94,221us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedLoanObligationsMember		  
Total 94,221us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedLoanObligationsMember		 94,236us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedLoanObligationsMember
Securities Available-for-Sale, Weighted Average Yield    
Securities not due at a single maturity date, Weighted Average Yield (as a percent) 2.85%osbc_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateWeightedAverageYield
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedLoanObligationsMember		  
Securities Available-for-Sale, Fair Value    
Securities not due at a single maturity date 93,017us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithoutSingleMaturityDateFairValue
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedLoanObligationsMember		  
Fair Value $ 93,017us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedLoanObligationsMember		 $ 92,209us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CollateralizedLoanObligationsMember