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Cover - USD ($)
$ in Millions
 12 Months Ended
Sep. 30, 2025
Nov. 14, 2025
Mar. 31, 2025
Cover [Abstract]      
Document Type 10-K    
Document Period End Date Sep. 30, 2025    
Current Fiscal Year End Date --09-30    
Document Transition Report false    
Securities Act File Number 1-33901    
Entity Registrant Name Oaktree Specialty Lending Corporation    
Entity Incorporation, State or Country Code DE    
Entity Tax Identification Number 26-1219283    
Entity Address, Address Line One 333 South Grand Avenue    
Entity Address, Address Line Two 28th Floor    
Entity Address, City or Town Los Angeles    
Entity Address, State or Province CA    
Entity Address, Postal Zip Code 90071    
City Area Code 213    
Local Phone Number 830-6300    
Title of 12(b) Security Common Stock, par value $0.01 per share    
Trading Symbol OCSL    
Security Exchange Name NASDAQ    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Large Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
ICFR Auditor Attestation Flag true    
Document Financial Statement Error Correction [Flag] false    
Entity Shell Company false    
Entity Public Float     $ 1,231.0
Entity Common Stock, Shares Outstanding   88,085,523  
Documents Incorporated by Reference
DOCUMENTS INCORPORATED BY REFERENCE

Portions of the registrant’s definitive Proxy Statement relating to the registrant’s 2026 Annual Meeting of Stockholders, to be filed with the Securities and Exchange Commission, or the SEC, within 120 days following the end of the Company’s fiscal year, are incorporated by reference in Part III of this Annual Report on Form 10-K as indicated herein.
    
Entity Central Index Key 0001414932    
Document Fiscal Year Focus 2025    
Document Fiscal Period Focus FY    
Amendment Flag false    
Document Annual Report true