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Regulatory & Capital Matters - Debt and Regulatory Capital (Details) (USD $)
Mar. 31, 2015
Dec. 31, 2014
Other regulatory capital information    
Subordinated debt $ 45,000,000us-gaap_SubordinatedDebt $ 45,000,000us-gaap_SubordinatedDebt
Bank of America credit facility    
Other regulatory capital information    
Subordinated debt 45,000,000us-gaap_SubordinatedDebt
/ us-gaap_DebtInstrumentAxis
= osbc_CreditFacilityMember
 
Debt obligation as a percent of Tier 2 regulatory capital 60.00%osbc_DebtObligationTierTwoRegulatoryCapitalQualifyingPercentage
/ us-gaap_DebtInstrumentAxis
= osbc_CreditFacilityMember
 
Junior subordinated debentures    
Other regulatory capital information    
Trust preferred proceeds qualified as Tier 1 regulatory capital 49,500,000osbc_ProceedsFromTrustPreferredSecuritiesQualifyingAsTierOneCapital
/ us-gaap_DebtInstrumentAxis
= us-gaap_JuniorSubordinatedDebtMember
56,600,000osbc_ProceedsFromTrustPreferredSecuritiesQualifyingAsTierOneCapital
/ us-gaap_DebtInstrumentAxis
= us-gaap_JuniorSubordinatedDebtMember
Trust preferred proceeds qualified as Tier 2 regulatory capital $ 7,100,000osbc_ProceedsFromTrustPreferredSecuritiesQualifyingAsTierTwoCapital
/ us-gaap_DebtInstrumentAxis
= us-gaap_JuniorSubordinatedDebtMember