Name:	osbc_BoardDesignatedCapitalToRiskWeightedAssets
Namespace Prefix:	osbc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

Board-designated threshold for Tier 1 Leverage Capital divided by average assets as defined in the regulations.

No definition available.

Name:	osbc_BoardDesignatedTierOneLeverageCapitalToAverageAssets
Namespace Prefix:	osbc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

The amount of deferment of cumulative effect on retained earnings due to adoption of new accounting standards.

No definition available.

Name:	osbc_DefermentOfCumulativeEffectOfImpactOnRetainedEarningsDueToAdoptionOfNewAccountingStandard
Namespace Prefix:	osbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Represents the basis point increase in the risk-based capital ratio over prior period.

No definition available.

Name:	osbc_IncreaseInCapitalToRiskWeightedAssetsRatio
Namespace Prefix:	osbc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

Represents the basis point increase in the Tier One leverage capital over prior period.

No definition available.

Name:	osbc_IncreaseInTierOneLeverageCapital
Namespace Prefix:	osbc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

The amount of phase out the impact of allowance for credit loss.

No definition available.

Name:	osbc_PhaseOutOfAllowanceForCreditLoss
Namespace Prefix:	osbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Ratio of total risk-based capital to risk-weighted assets as defined by regulatory framework.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 505
-Section 50
-Paragraph 1
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=117337116&loc=SL5958568-112826

Name:	us-gaap_CapitalToRiskWeightedAssets
Namespace Prefix:	us-gaap_
Data Type:	xbrli:pureItemType
Balance Type:	na
Period Type:	instant

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

Name:	us-gaap_ComplianceWithRegulatoryCapitalRequirementsUnderBankingRegulationsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Ratio of Tier 1 capital to average assets as defined by regulatory framework.