Amount of allowance for credit losses related to recorded investment.

No definition available.

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

- Definition

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

+ References

No definition available.

+ Details

Name:	osbc_ImpairedFinancingReceivableIncreaseDecreaseOfSpecificAllocationsWithinProvisionForLoanLosses
Namespace Prefix:	osbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

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

+ References

No definition available.

+ Details

Name:	osbc_RealEstateGross
Namespace Prefix:	osbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Fair value portion of asset recognized for present right to economic benefit.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 100
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482078/820-10-55-100

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 820
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482106/820-10-50-2

+ Details

Name:	us-gaap_AssetsFairValueDisclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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.

+ References

+ Details

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

- 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.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 310
-SubTopic 20
-Name Accounting Standards Codification
-Section 40
-Paragraph 7
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481628/310-20-40-7

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481990/310-10-45-3

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 11
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481962/310-10-50-11

+ Details

Name:	us-gaap_RealEstateAcquiredThroughForeclosure
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 942
-SubTopic 210
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(10)(4))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1

+ Details

Name:	us-gaap_RealEstateOwnedValuationAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Details

Name:	us-gaap_FairValueByMeasurementFrequencyAxis=us-gaap_FairValueMeasurementsNonrecurringMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=osbc_ImpairedLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByMeasurementBasisAxis=us-gaap_CarryingReportedAmountFairValueDisclosureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByAssetClassAxis=osbc_OtherRealEstateOwnedMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Fair Value Measurements - Nonrecurring (Details) - USD ($)	9 Months Ended	12 Months Ended
	Sep. 30, 2026	Dec. 31, 2025	Jun. 30, 2026	Sep. 30, 2025	Jun. 30, 2025	Dec. 31, 2024
Assets and liabilities measured at fair value
Carrying value of other real estate owned		$ 1,427,000	$ 622,000	$ 6,416,000	$ 6,486,000	$ 21,617,000
OREO Valuation allowance		632,000	$ 10,000	$ 632,000	$ 562,000	$ 1,862,000
Fair Value, Measurements, Nonrecurring
Assets and liabilities measured at fair value
Total	$ 20,709,000	35,857,000
Fair Value, Measurements, Nonrecurring \| Individually Evaluated Loans
Assets and liabilities measured at fair value
Total	20,709,000	34,430,000
Valuation allowance	7,300,000	6,400,000
Increase (decrease) of specific allocations within the provision for loan losses	837,000	747,000
Fair Value, Measurements, Nonrecurring \| Individually Evaluated Loans \| Reported Value Measurement
Assets and liabilities measured at fair value
Total	28,000,000	40,900,000
Fair Value, Measurements, Nonrecurring \| Other Real Estate Owned, net
Assets and liabilities measured at fair value
Total		1,427,000
Carrying value of other real estate owned		1,400,000
Outstanding balance		2,100,000
OREO Valuation allowance		632,000
Fair Value, Measurements, Nonrecurring \| Fair Value, Inputs, Level 3
Assets and liabilities measured at fair value
Total	20,709,000	35,857,000
Fair Value, Measurements, Nonrecurring \| Fair Value, Inputs, Level 3 \| Individually Evaluated Loans
Assets and liabilities measured at fair value
Total	$ 20,709,000	34,430,000
Fair Value, Measurements, Nonrecurring \| Fair Value, Inputs, Level 3 \| Other Real Estate Owned, net
Assets and liabilities measured at fair value
Total		$ 1,427,000