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Securities - Unrealized Loss Positions (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
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Dec. 31, 2014 security | Dec. 31, 2013 security
Number of Securities |  |
Less than 12 months in an unrealized loss position | 35 osbc_AvailableForSaleSecuritiesInUnrealizedLossPositionLessThan12MonthsQualitativeDisclosureNumberOfPositions | 27 osbc_AvailableForSaleSecuritiesInUnrealizedLossPositionLessThan12MonthsQualitativeDisclosureNumberOfPositions
Greater than 12 months in an unrealized loss position | 8 osbc_AvailableForSaleSecuritiesInUnrealizedLossPositions12MonthsOrLongerQualitativeDisclosureNumberOfPositions | 5 osbc_AvailableForSaleSecuritiesInUnrealizedLossPositions12MonthsOrLongerQualitativeDisclosureNumberOfPositions
Total | 43 us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1 | 32 us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1
Unrealized Losses |  |
Less than 12 months in an unrealized loss position | $ 4,764 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss | $ 6,633 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss
Greater than 12 months in an unrealized loss position | 2,591 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss | 532 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss
Total | 7,355 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss | 7,165 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss
Fair Value |  |
Less than 12 months in an unrealized loss position | 241,626 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue | 170,149 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Greater than 12 months in an unrealized loss position | 68,811 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue | 10,836 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue
Total | 310,437 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue | 180,985 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue
Number of Securities |  |
Less than 12 months in an unrealized loss position | 7 osbc_HeldToMaturitySecuritiesInUnrealizedLossPositionsLessThan12MonthsQualitativeDisclosureNumberOfPositions | 25 osbc_HeldToMaturitySecuritiesInUnrealizedLossPositionsLessThan12MonthsQualitativeDisclosureNumberOfPositions
Greater than 12 months in an unrealized loss position | 4 osbc_HeldToMaturitySecuritiesInUnrealizedLossPositions12MonthsOrLongerQualitativeDisclosureNumberOfPositions |
Total | 11 us-gaap_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions | 25 us-gaap_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions
Unrealized Losses |  |
Less than 12 months in an unrealized loss position | 457 us-gaap_HeldtomaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLoss | 2,931 us-gaap_HeldtomaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLoss
Greater than 12 months in an unrealized loss position | 982 us-gaap_HeldtomaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLoss |
Total | 1,439 us-gaap_HeldtomaturitySecuritiesContinuousUnrealizedLossPositionAggregateLoss | 2,931 us-gaap_HeldtomaturitySecuritiesContinuousUnrealizedLossPositionAggregateLoss
Fair Value |  |
Less than 12 months in an unrealized loss position | 49,302 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue | 175,766 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue
Greater than 12 months in an unrealized loss position | 46,283 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue |
Total | 95,585 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue | 175,766 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue
U.S. Treasury |  |
Number of Securities |  |
Less than 12 months in an unrealized loss position | 1 osbc_AvailableForSaleSecuritiesInUnrealizedLossPositionLessThan12MonthsQualitativeDisclosureNumberOfPositions / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasurySecuritiesMember | 1 osbc_AvailableForSaleSecuritiesInUnrealizedLossPositionLessThan12MonthsQualitativeDisclosureNumberOfPositions / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasurySecuritiesMember
Total | 1 us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1 / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasurySecuritiesMember | 1 us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1 / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasurySecuritiesMember
Unrealized Losses |  |
Less than 12 months in an unrealized loss position | 2 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasurySecuritiesMember | 5 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasurySecuritiesMember
Total | 2 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasurySecuritiesMember | 5 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasurySecuritiesMember
Fair Value |  |
Less than 12 months in an unrealized loss position | 1,527 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasurySecuritiesMember | 1,544 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasurySecuritiesMember
Total | 1,527 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasurySecuritiesMember | 1,544 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USTreasurySecuritiesMember
U.S. government agencies |  |
Number of Securities |  |
Greater than 12 months in an unrealized loss position | 1 osbc_AvailableForSaleSecuritiesInUnrealizedLossPositions12MonthsOrLongerQualitativeDisclosureNumberOfPositions / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember | 1 osbc_AvailableForSaleSecuritiesInUnrealizedLossPositions12MonthsOrLongerQualitativeDisclosureNumberOfPositions / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
Total | 1 us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1 / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember | 1 us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1 / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
Unrealized Losses |  |
Greater than 12 months in an unrealized loss position | 87 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember | 66 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
Total | 87 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember | 66 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
Fair Value |  |
Greater than 12 months in an unrealized loss position | 1,624 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember | 1,672 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
Total | 1,624 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember | 1,672 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USGovernmentCorporationsAndAgenciesSecuritiesMember
U.S. government agency mortgage-backed |  |
Number of Securities |  |
Less than 12 months in an unrealized loss position |  | 6 osbc_HeldToMaturitySecuritiesInUnrealizedLossPositionsLessThan12MonthsQualitativeDisclosureNumberOfPositions / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = osbc_MortgageBackedSecuritiesIssuedByUSGovernmentAgenciesMember
Total |  | 6 us-gaap_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = osbc_MortgageBackedSecuritiesIssuedByUSGovernmentAgenciesMember
Unrealized Losses |  |
Less than 12 months in an unrealized loss position |  | 73 us-gaap_HeldtomaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = osbc_MortgageBackedSecuritiesIssuedByUSGovernmentAgenciesMember
Total |  | 73 us-gaap_HeldtomaturitySecuritiesContinuousUnrealizedLossPositionAggregateLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = osbc_MortgageBackedSecuritiesIssuedByUSGovernmentAgenciesMember
Fair Value |  |
Less than 12 months in an unrealized loss position |  | 19,134 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = osbc_MortgageBackedSecuritiesIssuedByUSGovernmentAgenciesMember
Total |  | 19,134 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = osbc_MortgageBackedSecuritiesIssuedByUSGovernmentAgenciesMember
States and political subdivisions |  |
Number of Securities |  |
Less than 12 months in an unrealized loss position | 4 osbc_AvailableForSaleSecuritiesInUnrealizedLossPositionLessThan12MonthsQualitativeDisclosureNumberOfPositions / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 6 osbc_AvailableForSaleSecuritiesInUnrealizedLossPositionLessThan12MonthsQualitativeDisclosureNumberOfPositions / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Total | 4 us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1 / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 6 us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1 / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Unrealized Losses |  |
Less than 12 months in an unrealized loss position | 96 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 217 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Total | 96 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 217 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Fair Value |  |
Less than 12 months in an unrealized loss position | 4,896 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 4,625 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Total | 4,896 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember | 4,625 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_USStatesAndPoliticalSubdivisionsMember
Corporate bonds |  |
Number of Securities |  |
Less than 12 months in an unrealized loss position | 4 osbc_AvailableForSaleSecuritiesInUnrealizedLossPositionLessThan12MonthsQualitativeDisclosureNumberOfPositions / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember | 4 osbc_AvailableForSaleSecuritiesInUnrealizedLossPositionLessThan12MonthsQualitativeDisclosureNumberOfPositions / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember
Greater than 12 months in an unrealized loss position | 1 osbc_AvailableForSaleSecuritiesInUnrealizedLossPositions12MonthsOrLongerQualitativeDisclosureNumberOfPositions / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember | 2 osbc_AvailableForSaleSecuritiesInUnrealizedLossPositions12MonthsOrLongerQualitativeDisclosureNumberOfPositions / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember
Total | 5 us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1 / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember | 6 us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1 / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember
Unrealized Losses |  |
Less than 12 months in an unrealized loss position | 486 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember | 429 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember
Greater than 12 months in an unrealized loss position | 81 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember | 219 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember
Total | 567 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember | 648 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember
Fair Value |  |
Less than 12 months in an unrealized loss position | 15,246 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember | 10,493 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember
Greater than 12 months in an unrealized loss position | 1,921 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember | 2,796 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember
Total | 17,167 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember | 13,289 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CorporateBondSecuritiesMember
Collateralized mortgage obligations |  |
Number of Securities |  |
Less than 12 months in an unrealized loss position | 5 osbc_AvailableForSaleSecuritiesInUnrealizedLossPositionLessThan12MonthsQualitativeDisclosureNumberOfPositions / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 5 osbc_AvailableForSaleSecuritiesInUnrealizedLossPositionLessThan12MonthsQualitativeDisclosureNumberOfPositions / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Greater than 12 months in an unrealized loss position | 3 osbc_AvailableForSaleSecuritiesInUnrealizedLossPositions12MonthsOrLongerQualitativeDisclosureNumberOfPositions / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember |
Total | 8 us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1 / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 5 us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1 / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Unrealized Losses |  |
Less than 12 months in an unrealized loss position | 900 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 3,146 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Greater than 12 months in an unrealized loss position | 1,232 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember |
Total | 2,132 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 3,146 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Fair Value |  |
Less than 12 months in an unrealized loss position | 38,284 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 54,021 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Greater than 12 months in an unrealized loss position | 21,604 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember |
Total | 59,888 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 54,021 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Number of Securities |  |
Less than 12 months in an unrealized loss position | 7 osbc_HeldToMaturitySecuritiesInUnrealizedLossPositionsLessThan12MonthsQualitativeDisclosureNumberOfPositions / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 19 osbc_HeldToMaturitySecuritiesInUnrealizedLossPositionsLessThan12MonthsQualitativeDisclosureNumberOfPositions / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Greater than 12 months in an unrealized loss position | 4 osbc_HeldToMaturitySecuritiesInUnrealizedLossPositions12MonthsOrLongerQualitativeDisclosureNumberOfPositions / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember |
Total | 11 us-gaap_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 19 us-gaap_HeldToMaturitySecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Unrealized Losses |  |
Less than 12 months in an unrealized loss position | 457 us-gaap_HeldtomaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 2,858 us-gaap_HeldtomaturitySecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAggregateLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Greater than 12 months in an unrealized loss position | 982 us-gaap_HeldtomaturitySecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAggregateLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember |
Total | 1,439 us-gaap_HeldtomaturitySecuritiesContinuousUnrealizedLossPositionAggregateLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 2,858 us-gaap_HeldtomaturitySecuritiesContinuousUnrealizedLossPositionAggregateLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Fair Value |  |
Less than 12 months in an unrealized loss position | 49,302 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 156,632 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Greater than 12 months in an unrealized loss position | 46,283 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember |
Total | 95,585 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember | 156,632 us-gaap_HeldToMaturitySecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedMortgageObligationsMember
Asset-backed securities |  |
Number of Securities |  |
Less than 12 months in an unrealized loss position | 9 osbc_AvailableForSaleSecuritiesInUnrealizedLossPositionLessThan12MonthsQualitativeDisclosureNumberOfPositions / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember | 11 osbc_AvailableForSaleSecuritiesInUnrealizedLossPositionLessThan12MonthsQualitativeDisclosureNumberOfPositions / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Greater than 12 months in an unrealized loss position | 3 osbc_AvailableForSaleSecuritiesInUnrealizedLossPositions12MonthsOrLongerQualitativeDisclosureNumberOfPositions / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember | 2 osbc_AvailableForSaleSecuritiesInUnrealizedLossPositions12MonthsOrLongerQualitativeDisclosureNumberOfPositions / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Total | 12 us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1 / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember | 13 us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1 / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Unrealized Losses |  |
Less than 12 months in an unrealized loss position | 1,077 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember | 2,836 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Greater than 12 months in an unrealized loss position | 1,191 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember | 247 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Total | 2,268 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember | 3,083 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Fair Value |  |
Less than 12 months in an unrealized loss position | 99,286 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember | 99,466 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Greater than 12 months in an unrealized loss position | 43,662 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember | 6,368 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Total | 142,948 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember | 105,834 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_AssetBackedSecuritiesMember
Collateralized loan obligations |  |
Number of Securities |  |
Less than 12 months in an unrealized loss position | 12 osbc_AvailableForSaleSecuritiesInUnrealizedLossPositionLessThan12MonthsQualitativeDisclosureNumberOfPositions / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedLoanObligationsMember |
Total | 12 us-gaap_AvailableforsaleSecuritiesInUnrealizedLossPositionsQualitativeDisclosureNumberOfPositions1 / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedLoanObligationsMember |
Unrealized Losses |  |
Less than 12 months in an unrealized loss position | 2,203 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedLoanObligationsMember |
Total | 2,203 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedLoanObligationsMember |
Fair Value |  |
Less than 12 months in an unrealized loss position | 82,387 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedLoanObligationsMember |
Total | $ 82,387 us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue / us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis = us-gaap_CollateralizedLoanObligationsMember |

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