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Financial Instruments with Off-Balance Sheet Risk and Derivative Transactions - Interest Rate Derivatives (Details) (Not designated, USD $) | 3 Months Ended | 12 Months Ended
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Mar. 31, 2015 item | Dec. 31, 2014 item
Financial Instruments with Off-Balance Sheet Risk and Derivative Transactions |  |
Asset Derivatives, Fair Value | $ 362,000 us-gaap_DerivativeAssets | $ 173,000 us-gaap_DerivativeAssets
Liability Derivatives, Fair Value | 66,000 us-gaap_DerivativeLiabilities | 30,000 us-gaap_DerivativeLiabilities
Interest rate swap agreements net of swap credit valuation |  |
Financial Instruments with Off-Balance Sheet Risk and Derivative Transactions |  |
Investment securities pledged with financial institutions, dollars | 1,300,000 us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesForOtherDebtFacilities / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | 3,000,000 us-gaap_PledgedFinancialInstrumentsNotSeparatelyReportedSecuritiesForOtherDebtFacilities / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember
Number of correspondent financial institutions where investment securities are pledged | 2 osbc_NumberOfFinancialInstitutionsInvestmentSecuritiesPledged / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | 3 osbc_NumberOfFinancialInstitutionsInvestmentSecuritiesPledged / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember
Weighted average maturity | 2 years 10 months 24 days | 2 years 8 months 12 days
Notional or Contractual Amount | 13,123,000 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | 16,334,000 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember
Asset Derivatives, Fair Value | 66,000 us-gaap_DerivativeAssets / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | 30,000 us-gaap_DerivativeAssets / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember
Liability Derivatives, Fair Value | 66,000 us-gaap_DerivativeLiabilities / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | 30,000 us-gaap_DerivativeLiabilities / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_InterestRateSwapMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember
Commitments |  |
Financial Instruments with Off-Balance Sheet Risk and Derivative Transactions |  |
Notional or Contractual Amount | 222,684,000 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = osbc_DerivativeCommitmentsMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | 201,946,000 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = osbc_DerivativeCommitmentsMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember
Asset Derivatives, Fair Value | 296,000 us-gaap_DerivativeAssets / us-gaap_DerivativeInstrumentRiskAxis = osbc_DerivativeCommitmentsMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | 143,000 us-gaap_DerivativeAssets / us-gaap_DerivativeInstrumentRiskAxis = osbc_DerivativeCommitmentsMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember
Forward MBS contracts and forward loan contracts |  |
Financial Instruments with Off-Balance Sheet Risk and Derivative Transactions |  |
Notional or Contractual Amount | $ 28,654,000 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForwardContractsMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember | $ 14,000,000 invest_DerivativeNotionalAmount / us-gaap_DerivativeInstrumentRiskAxis = us-gaap_ForwardContractsMember / us-gaap_HedgingDesignationAxis = us-gaap_NondesignatedMember

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- Definition Aggregate notional amount specified by the derivative(s). Expressed as an absolute value. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher SEC -Name Regulation S-X (SX) -Number 210 -Article 12 -Section 13 -Sentence Column B + Details
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- Definition Represents the number of financial institutions where investment securities are pledged. + References No definition available. + Details
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- Definition Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes assets not subject to a master netting arrangement and not elected to be offset. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13433-108611 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13495-108611 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=34720828&loc=SL20225862-175312 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=36459792&loc=SL20226008-175313 + Details
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- Definition Average remaining period until maturity of the derivative contract, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days. + References No definition available. + Details
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- Definition Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset. Includes liabilities not subject to a master netting arrangement and not elected to be offset. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 10 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13433-108611 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 5 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41228-113958 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 825 -SubTopic 10 -Section 50 -Paragraph 15 -URI http://asc.fasb.org/extlink&oid=35750535&loc=d3e13495-108611 Reference 4: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 50 -Paragraph 3 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=34720828&loc=SL20225862-175312 Reference 5: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 815 -SubTopic 10 -Section 45 -Paragraph 6 -URI http://asc.fasb.org/extlink&oid=6945355&loc=d3e41271-113958 Reference 6: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 20 -Section 55 -Paragraph 10 -URI http://asc.fasb.org/extlink&oid=36459792&loc=SL20226008-175313 + Details
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- Definition The carrying amount as of the date of the latest financial statement presented of securities which are owned but transferred to serve as collateral for debt facilities other than FHLB loans, repurchase agreements and letter of credit arrangements, and for which the transferee does not have the right by contract or custom to sell or re-pledge them to an unrelated party. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 860 -SubTopic 30 -Section 50 -Paragraph 1A -Subparagraph (b)(1)(i) -URI http://asc.fasb.org/extlink&oid=34720971&loc=SL6224234-111729 + Details
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