v3.25.2

Portfolio Investments - Schedule of Significant Unobservable Inputs for Level 3 Investments (Details) $ in Thousands | Jun. 30, 2025 USD ($) | Sep. 30, 2024 USD ($)
--- | --- | ---
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value | $ 2,809,377 | $ 3,021,279
Senior secured debt |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value | 2,343,411 | 2,574,937
Subordinated debt |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value | 138,122 | 109,921
Debt investments in the JVs |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value | 159,720 | 161,552
Level 3 |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value | 2,485,322 | 2,723,732
Level 3 | Senior secured debt |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value | 2,076,845 | 2,320,310
Level 3 | Senior secured debt | Market Yield | Market Yield |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value | $ 1,988,077 | $ 2,044,221
Level 3 | Senior secured debt | Market Yield | Market Yield | Minimum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value, measurement input | 0.070 | 0.057
Level 3 | Senior secured debt | Market Yield | Market Yield | Maximum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value, measurement input | 0.400 | 0.310
Level 3 | Senior secured debt | Market Yield | Market Yield | Weighted Average |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value, measurement input | 0.112 | 0.120
Level 3 | Senior secured debt | Enterprise Value | Revenue Multiple |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value | $ 44,161 | $ 52,857
Level 3 | Senior secured debt | Enterprise Value | Revenue Multiple | Minimum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value, measurement input | 0.5 | 2.0
Level 3 | Senior secured debt | Enterprise Value | Revenue Multiple | Maximum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value, measurement input | 5.5 | 5.5
Level 3 | Senior secured debt | Enterprise Value | Revenue Multiple | Weighted Average |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value, measurement input | 2.8 | 3.7
Level 3 | Senior secured debt | Enterprise Value | EBITDA Multiple |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value | $ 31,985 | $ 26,927
Level 3 | Senior secured debt | Enterprise Value | EBITDA Multiple | Minimum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value, measurement input | 2.6 | 4.5
Level 3 | Senior secured debt | Enterprise Value | EBITDA Multiple | Maximum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value, measurement input | 8.2 | 7.5
Level 3 | Senior secured debt | Enterprise Value | EBITDA Multiple | Weighted Average |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value, measurement input | 6.7 | 5.7
Level 3 | Senior secured debt | Transaction Precedent | Transaction Price |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value | $ 12,622 | $ 33,272
Level 3 | Senior secured debt | Broker Quotations | Broker Quoted Price |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value |  | 163,033
Level 3 | Subordinated debt | Market Yield | Market Yield |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value | $ 97,778 | $ 93,794
Level 3 | Subordinated debt | Market Yield | Market Yield | Minimum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value, measurement input | 0.050 | 0.050
Level 3 | Subordinated debt | Market Yield | Market Yield | Maximum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value, measurement input | 0.120 | 0.450
Level 3 | Subordinated debt | Market Yield | Market Yield | Weighted Average |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value, measurement input | 0.093 | 0.101
Level 3 | Debt investments in the JVs | Enterprise Value |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value | $ 159,720 | $ 161,552
Level 3 | Preferred & Common Equity | Enterprise Value | Revenue Multiple |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value | $ 64,170 | $ 80,720
Level 3 | Preferred & Common Equity | Enterprise Value | Revenue Multiple | Minimum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value, measurement input | 0.3 | 0.3
Level 3 | Preferred & Common Equity | Enterprise Value | Revenue Multiple | Maximum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value, measurement input | 5.5 | 7.2
Level 3 | Preferred & Common Equity | Enterprise Value | Revenue Multiple | Weighted Average |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value, measurement input | 0.6 | 2.0
Level 3 | Preferred & Common Equity | Enterprise Value | EBITDA Multiple |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value | $ 85,313 | $ 66,106
Level 3 | Preferred & Common Equity | Enterprise Value | EBITDA Multiple | Minimum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value, measurement input | 2.6 | 2.9
Level 3 | Preferred & Common Equity | Enterprise Value | EBITDA Multiple | Maximum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value, measurement input | 17.4 | 15.0
Level 3 | Preferred & Common Equity | Enterprise Value | EBITDA Multiple | Weighted Average |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value, measurement input | 10.7 | 10.2
Level 3 | Preferred & Common Equity | Enterprise Value | Asset Multiple |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value | $ 1,496 | $ 1,250
Level 3 | Preferred & Common Equity | Enterprise Value | Asset Multiple | Minimum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value, measurement input | 1.4 | 1.0
Level 3 | Preferred & Common Equity | Enterprise Value | Asset Multiple | Maximum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value, measurement input | 1.6 | 1.4
Level 3 | Preferred & Common Equity | Enterprise Value | Asset Multiple | Weighted Average |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value, measurement input | 1.5 | 1.4

X
- 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 Reference 1: http://www.xbrl.org/2003/role/exampleRef -Topic 805 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 8 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479328/805-10-50-8 Reference 2: http://www.xbrl.org/2003/role/exampleRef -Topic 805 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 43 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479303/805-10-55-43 Reference 3: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (bbb)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 4: http://www.xbrl.org/2003/role/exampleRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 55 -Paragraph 103 -Publisher FASB -URI https://asc.fasb.org/1943274/2147482078/820-10-55-103 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (bbb)(2)(i) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 6: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (bbb)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 + Details
Name: | us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisValuationTechniquesLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition Value of input used to measure investment owned by investment company. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (bbb)(2)(i) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (bbb)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 + Details
Name: | us-gaap_InvestmentCompanyInvestmentOwnedFairValueMeasurementInput
Namespace Prefix: | us-gaap_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | instant

X
- Definition Fair value of investment in security owned. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.6-04(11)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479170/946-210-S99-1 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 210 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478795/946-210-50-6 Reference 3: http://www.xbrl.org/2003/role/exampleRef -Topic 946 -SubTopic 210 -Name Accounting Standards Codification -Section 55 -Paragraph 1 -Publisher FASB -URI https://asc.fasb.org/1943274/2147477439/946-210-55-1 Reference 4: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 210 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Subparagraph (a)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478795/946-210-50-6 Reference 5: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 210 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Subparagraph (a)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478795/946-210-50-6 Reference 6: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 210 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (a)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478795/946-210-50-1 Reference 7: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 210 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (b)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478795/946-210-50-1 Reference 8: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 210 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (b)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478795/946-210-50-1 Reference 9: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 210 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (c)(2) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478795/946-210-50-1 Reference 10: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 210 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Publisher FASB -URI https://asc.fasb.org/1943274/2147478795/946-210-50-2 Reference 11: http://www.xbrl.org/2003/role/exampleRef -Topic 946 -SubTopic 830 -Name Accounting Standards Codification -Section 55 -Paragraph 12 -Publisher FASB -URI https://asc.fasb.org/1943274/2147479168/946-830-55-12 Reference 12: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 946 -SubTopic 210 -Name Accounting Standards Codification -Section 50 -Paragraph 6 -Subparagraph (a)(3) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478795/946-210-50-6 Reference 13: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 10 -Name Accounting Standards Codification -Section S99 -Paragraph 3 -Subparagraph (SX 210.6-03(d)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479886/946-10-S99-3 Reference 14: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.6-04(1)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479170/946-210-S99-1 Reference 15: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.6-04(2)(a)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479170/946-210-S99-1 Reference 16: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.6-04(2)(b)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147479170/946-210-S99-1 Reference 17: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 320 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.12-12(Column C)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477271/946-320-S99-1 Reference 18: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 320 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.12-12(Column A)(Footnote 1)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477271/946-320-S99-1 Reference 19: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 320 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.12-12(Column C)(Footnote 5)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477271/946-320-S99-1 Reference 20: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 320 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.12-12(Column C)(Footnote 6)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477271/946-320-S99-1 Reference 21: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 320 -Name Accounting Standards Codification -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-12B(Column C)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477271/946-320-S99-3 Reference 22: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 320 -Name Accounting Standards Codification -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-12B(Column C)(Footnote 2)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477271/946-320-S99-3 Reference 23: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 320 -Name Accounting Standards Codification -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-12B(Column A)(Footnote 6)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477271/946-320-S99-3 Reference 24: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 320 -Name Accounting Standards Codification -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-12B(Column A)(Footnote 7)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477271/946-320-S99-3 Reference 25: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 320 -Name Accounting Standards Codification -Section S99 -Paragraph 3 -Subparagraph (SX 210.12-12B(Column C)(Footnote 9)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477271/946-320-S99-3 Reference 26: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 320 -Name Accounting Standards Codification -Section S99 -Paragraph 6 -Subparagraph (SX 210.12-14(Column F)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477271/946-320-S99-6 Reference 27: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 320 -Name Accounting Standards Codification -Section S99 -Paragraph 6 -Subparagraph (SX 210.12-14(Column A)(Footnote 1)(b)(2)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477271/946-320-S99-6 Reference 28: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 320 -Name Accounting Standards Codification -Section S99 -Paragraph 6 -Subparagraph (SX 210.12-14(Column A)(Footnote 1)(b)(5)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477271/946-320-S99-6 Reference 29: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 320 -Name Accounting Standards Codification -Section S99 -Paragraph 6 -Subparagraph (SX 210.12-14(Column F)(Footnote 4)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477271/946-320-S99-6 Reference 30: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 320 -Name Accounting Standards Codification -Section S99 -Paragraph 6 -Subparagraph (SX 210.12-14(Column F)(Footnote 7)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147477271/946-320-S99-6 + Details
Name: | us-gaap_InvestmentOwnedAtFairValue
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=ocsl_DebtSecuritiesSeniorSecuredMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=us-gaap_SubordinatedDebtMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=ocsl_DebtSecuritiesJVMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

X
- Details
Name: | us-gaap_ValuationTechniqueAxis=us-gaap_MarketApproachValuationTechniqueMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MeasurementInputTypeAxis=ocsl_MeasurementInputMarketYieldMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | srt_RangeAxis=srt_WeightedAverageMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ValuationTechniqueAxis=ocsl_ValuationEnterpriseValueMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputRevenueMultipleMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputEbitdaMultipleMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ValuationTechniqueAxis=ocsl_ValuationTransactionPrecedentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MeasurementInputTypeAxis=ocsl_MeasurementInputTransactionPriceMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_ValuationTechniqueAxis=ocsl_ValuationBrokerQuotationsMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MeasurementInputTypeAxis=ocsl_MeasurementInputBrokerQuotedPriceMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_FinancialInstrumentAxis=ocsl_EquitySecuritiesPreferredCommonMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

X
- Details
Name: | us-gaap_MeasurementInputTypeAxis=ocsl_MeasurementInputAssetMultipleMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |