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Fair Value Measurements - Nonrecurring (Details) (USD $)	3 Months Ended	12 Months Ended
	Mar. 31, 2015	Dec. 31, 2014	Mar. 31, 2014	Dec. 31, 2013
Assets and liabilities measured at fair value
Valuation allowance	$ 1,784,000us-gaap_ImpairedFinancingReceivableRelatedAllowance	$ 278,000us-gaap_ImpairedFinancingReceivableRelatedAllowance
Carrying value of other real estate owned	35,461,000us-gaap_RealEstateAcquiredThroughForeclosure	31,982,000us-gaap_RealEstateAcquiredThroughForeclosure	40,220,000us-gaap_RealEstateAcquiredThroughForeclosure	41,537,000us-gaap_RealEstateAcquiredThroughForeclosure
OREO Valuation allowance	19,456,000us-gaap_RealEstateOwnedValuationAllowance	19,229,000us-gaap_RealEstateOwnedValuationAllowance	19,484,000us-gaap_RealEstateOwnedValuationAllowance	22,284,000us-gaap_RealEstateOwnedValuationAllowance
Nonrecurring | Impaired loans
Assets and liabilities measured at fair value
Valuation allowance	1,800,000us-gaap_ImpairedFinancingReceivableRelatedAllowance / us-gaap_FairValueByAssetClassAxis = osbc_ImpairedLoansMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember	278,000us-gaap_ImpairedFinancingReceivableRelatedAllowance / us-gaap_FairValueByAssetClassAxis = osbc_ImpairedLoansMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember
Decrease of specific allocations within the provision for loan losses	1,500,000osbc_ImpairedFinancingReceivableDecreaseOfSpecificAllocationsWithinProvisionForLoanLosses / us-gaap_FairValueByAssetClassAxis = osbc_ImpairedLoansMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember	2,100,000osbc_ImpairedFinancingReceivableDecreaseOfSpecificAllocationsWithinProvisionForLoanLosses / us-gaap_FairValueByAssetClassAxis = osbc_ImpairedLoansMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember
Nonrecurring | Impaired loans | Carrying Amount
Assets and liabilities measured at fair value
Total	6,600,000us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = osbc_ImpairedLoansMember / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember	842,000us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = osbc_ImpairedLoansMember / us-gaap_FairValueByMeasurementBasisAxis = us-gaap_CarryingReportedAmountFairValueDisclosureMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember
Nonrecurring | Other real estate owned
Assets and liabilities measured at fair value
Carrying value of other real estate owned	35,500,000us-gaap_RealEstateAcquiredThroughForeclosure / us-gaap_FairValueByAssetClassAxis = osbc_OtherRealEstateOwnedMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember	32,000,000us-gaap_RealEstateAcquiredThroughForeclosure / us-gaap_FairValueByAssetClassAxis = osbc_OtherRealEstateOwnedMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember
Outstanding balance	56,800,000osbc_RealEstateGross / us-gaap_FairValueByAssetClassAxis = osbc_OtherRealEstateOwnedMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember	53,000,000osbc_RealEstateGross / us-gaap_FairValueByAssetClassAxis = osbc_OtherRealEstateOwnedMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember
OREO Valuation allowance	19,500,000us-gaap_RealEstateOwnedValuationAllowance / us-gaap_FairValueByAssetClassAxis = osbc_OtherRealEstateOwnedMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember	19,200,000us-gaap_RealEstateOwnedValuationAllowance / us-gaap_FairValueByAssetClassAxis = osbc_OtherRealEstateOwnedMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember
OREO participations	1,800,000osbc_RealEstateOwnedParticipations / us-gaap_FairValueByAssetClassAxis = osbc_OtherRealEstateOwnedMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember	1,800,000osbc_RealEstateOwnedParticipations / us-gaap_FairValueByAssetClassAxis = osbc_OtherRealEstateOwnedMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember
Nonrecurring | Level 3
Assets and liabilities measured at fair value
Total	40,237,000us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember	32,546,000us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember
Nonrecurring | Level 3 | Impaired loans
Assets and liabilities measured at fair value
Total	4,776,000us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = osbc_ImpairedLoansMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember	564,000us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = osbc_ImpairedLoansMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember
Nonrecurring | Level 3 | Other real estate owned
Assets and liabilities measured at fair value
Total	35,461,000us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = osbc_OtherRealEstateOwnedMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember	31,982,000us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = osbc_OtherRealEstateOwnedMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_FairValueInputsLevel3Member / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember
Nonrecurring | Total
Assets and liabilities measured at fair value
Total	40,237,000us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember	32,546,000us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember
Nonrecurring | Total | Impaired loans
Assets and liabilities measured at fair value
Total	4,776,000us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = osbc_ImpairedLoansMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember	564,000us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = osbc_ImpairedLoansMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember
Nonrecurring | Total | Other real estate owned
Assets and liabilities measured at fair value
Total	$ 35,461,000us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = osbc_OtherRealEstateOwnedMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember	$ 31,982,000us-gaap_AssetsFairValueDisclosure / us-gaap_FairValueByAssetClassAxis = osbc_OtherRealEstateOwnedMember / us-gaap_FairValueByFairValueHierarchyLevelAxis = us-gaap_EstimateOfFairValueFairValueDisclosureMember / us-gaap_FairValueByMeasurementFrequencyAxis = us-gaap_FairValueMeasurementsNonrecurringMember

X

- Definition

Represents the amount of increase (decrease) in specific allocations within the provision for loan losses for impaired financing receivables.

+ References

No definition available.

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X

- Definition

Represents the gross carrying amount of real estate owned before the valuation allowance.

+ References

No definition available.

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X

- Definition

Represents the amount of the participations in the real estate owned.

+ References

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X

- Definition

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

+ References

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-Publisher FASB

-Name Accounting Standards Codification

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-SubTopic 10

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- Definition

Amount of allowance for credit losses related to recorded investment.

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- Definition

Carrying amount as of the balance sheet date of land and buildings obtained through foreclosure proceedings or defeasance in full or partial satisfaction of a debt arrangement.

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-Publisher FASB

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For each period for which an income statement is required, disclosure of the changes in the allowance, including balances at end of period.

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