v3.23.3

Portfolio Investments - Significant Unobservable Inputs for Level 3 Investments (Details) $ in Thousands | Sep. 30, 2023 USD ($) | Sep. 30, 2022 USD ($)
--- | --- | ---
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value | $ 2,892,420 | $ 2,494,111
Senior secured debt |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value | 2,501,385 | 2,166,409
Debt investments in the JVs |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value | 162,673 | 146,533
Level 3 |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value | 2,619,912 | 2,169,475
Level 3 | Senior secured debt |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value | 2,292,691 | 1,910,606
Level 3 | Senior secured debt | Valuation, Market Approach | Measurement Input, Market Yield |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value | $ 1,904,140 | $ 1,599,148
Level 3 | Senior secured debt | Valuation, Market Approach | Measurement Input, Market Yield | Minimum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investment Company, Investment Owned, Measurement Input | 0.090 | 0.090
Level 3 | Senior secured debt | Valuation, Market Approach | Measurement Input, Market Yield | Maximum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investment Company, Investment Owned, Measurement Input | 0.320 | 0.300
Level 3 | Senior secured debt | Valuation, Market Approach | Measurement Input, Market Yield | Weighted Average |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investment Company, Investment Owned, Measurement Input | 0.147 | 0.137
Level 3 | Senior secured debt | Valuation, Enterprise Value | Measurement Input, EBITDA Multiple |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value | $ 64,802 | $ 14,333
Level 3 | Senior secured debt | Valuation, Enterprise Value | Measurement Input, EBITDA Multiple | Minimum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investment Company, Investment Owned, Measurement Input | 3.0 | 5.0
Level 3 | Senior secured debt | Valuation, Enterprise Value | Measurement Input, EBITDA Multiple | Maximum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investment Company, Investment Owned, Measurement Input | 6.0 | 7.0
Level 3 | Senior secured debt | Valuation, Enterprise Value | Measurement Input, EBITDA Multiple | Weighted Average |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investment Company, Investment Owned, Measurement Input | 4.6 | 6.0
Level 3 | Senior secured debt | Valuation, Transaction Precedent | Measurement Input, Transaction Price |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value | $ 33,816 |
Level 3 | Senior secured debt | Valuation, Broker Quotations | Measurement Input, Transaction Price |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value |  | $ 297,125
Level 3 | Senior secured debt | Valuation, Broker Quotations | Measurement Input, Broker Quoted Price |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value | 289,933 |
Level 3 | Subordinated Debt | Valuation, Market Approach | Measurement Input, Market Yield |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value | $ 22,881 | $ 12,855
Level 3 | Subordinated Debt | Valuation, Market Approach | Measurement Input, Market Yield | Minimum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investment Company, Investment Owned, Measurement Input | 0.100 | 0.100
Level 3 | Subordinated Debt | Valuation, Market Approach | Measurement Input, Market Yield | Maximum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investment Company, Investment Owned, Measurement Input | 0.220 | 0.190
Level 3 | Subordinated Debt | Valuation, Market Approach | Measurement Input, Market Yield | Weighted Average |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investment Company, Investment Owned, Measurement Input | 0.112 | 0.138
Level 3 | Subordinated Debt | Valuation, Broker Quotations | Measurement Input, Broker Quoted Price |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value | $ 4,170 |
Level 3 | Debt investments in the JVs | Valuation, Enterprise Value |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value | 162,673 | $ 146,533
Level 3 | Equity Securities, Preferred & Common | Valuation, Enterprise Value | Measurement Input, Revenue Multiple |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value | $ 32,318 | $ 61,693
Level 3 | Equity Securities, Preferred & Common | Valuation, Enterprise Value | Measurement Input, Revenue Multiple | Minimum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investment Company, Investment Owned, Measurement Input | 0.4 | 0.4
Level 3 | Equity Securities, Preferred & Common | Valuation, Enterprise Value | Measurement Input, Revenue Multiple | Maximum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investment Company, Investment Owned, Measurement Input | 3.2 | 10.1
Level 3 | Equity Securities, Preferred & Common | Valuation, Enterprise Value | Measurement Input, Revenue Multiple | Weighted Average |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investment Company, Investment Owned, Measurement Input | 0.5 | 4.3
Level 3 | Equity Securities, Preferred & Common | Valuation, Enterprise Value | Measurement Input, EBITDA Multiple |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value | $ 103,661 | $ 36,913
Level 3 | Equity Securities, Preferred & Common | Valuation, Enterprise Value | Measurement Input, EBITDA Multiple | Minimum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investment Company, Investment Owned, Measurement Input | 1.7 | 3.0
Level 3 | Equity Securities, Preferred & Common | Valuation, Enterprise Value | Measurement Input, EBITDA Multiple | Maximum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investment Company, Investment Owned, Measurement Input | 15.1 | 20.0
Level 3 | Equity Securities, Preferred & Common | Valuation, Enterprise Value | Measurement Input, EBITDA Multiple | Weighted Average |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investment Company, Investment Owned, Measurement Input | 8.8 | 11.4
Level 3 | Equity Securities, Preferred & Common | Valuation, Enterprise Value | Measurement Input, Asset Multiple |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value | $ 1,097 | $ 3
Level 3 | Equity Securities, Preferred & Common | Valuation, Enterprise Value | Measurement Input, Asset Multiple | Minimum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investment Company, Investment Owned, Measurement Input | 1.0 | 0.9
Level 3 | Equity Securities, Preferred & Common | Valuation, Enterprise Value | Measurement Input, Asset Multiple | Maximum |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investment Company, Investment Owned, Measurement Input | 1.4 | 1.1
Level 3 | Equity Securities, Preferred & Common | Valuation, Enterprise Value | Measurement Input, Asset Multiple | Weighted Average |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investment Company, Investment Owned, Measurement Input | 1.4 | 1.0
Level 3 | Equity Securities, Preferred & Common | Valuation, Transaction Precedent | Measurement Input, Transaction Price |  |
Fair Value Measurement Inputs and Valuation Techniques [Line Items] |  |
Investments at fair value | $ 421 | $ 872

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 No definition available. + 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) -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 50 -Paragraph 6 -Subparagraph (c) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480524/946-210-50-6 Reference 2: 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//1943274/2147480493/946-210-55-1 Reference 3: 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//1943274/2147480524/946-210-50-6 Reference 4: 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//1943274/2147480524/946-210-50-6 Reference 5: 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//1943274/2147480524/946-210-50-1 Reference 6: 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//1943274/2147480524/946-210-50-1 Reference 7: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 210 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (b)(2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480524/946-210-50-1 Reference 8: http://www.xbrl.org/2003/role/disclosureRef -Topic 946 -SubTopic 210 -Name Accounting Standards Codification -Section 50 -Paragraph 1 -Subparagraph (c)(2) -Publisher FASB -URI https://asc.fasb.org//1943274/2147480524/946-210-50-1 Reference 9: 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//1943274/2147480524/946-210-50-2 Reference 10: 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//1943274/2147480167/946-830-55-12 Reference 11: 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//1943274/2147480524/946-210-50-6 Reference 12: 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//1943274/2147479617/946-210-S99-1 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//1943274/2147479617/946-210-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//1943274/2147479617/946-210-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//1943274/2147479617/946-210-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//1943274/2147480032/946-320-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//1943274/2147480032/946-320-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//1943274/2147480032/946-320-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//1943274/2147480032/946-320-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//1943274/2147480032/946-320-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//1943274/2147480032/946-320-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//1943274/2147480032/946-320-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//1943274/2147480032/946-320-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//1943274/2147480032/946-320-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//1943274/2147480032/946-320-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//1943274/2147480032/946-320-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//1943274/2147480032/946-320-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//1943274/2147480032/946-320-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//1943274/2147480032/946-320-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=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_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=us-gaap_SubordinatedDebtMember
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=us-gaap_MeasurementInputRevenueMultipleMember
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: |