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N-2 - USD ($) $ / shares in Units, $ in Millions	9 Months Ended
N-2 - USD ($) $ / shares in Units, $ in Millions	Jun. 30, 2026	Mar. 31, 2026	Sep. 30, 2025	Jun. 30, 2025	Mar. 31, 2025	Sep. 30, 2024
Cover [Abstract]
Entity Central Index Key	0001414932
Amendment Flag	false
Securities Act File Number	1-33901
Document Type	10-Q
Entity Registrant Name	Oaktree Specialty Lending Corporation
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
Entity Emerging Growth Company	false
Financial Highlights [Abstract]
Senior Securities Amount	$ 1,454.7			$ 1,466.1
General Description of Registrant [Abstract]
NAV Per Share	$ 15.70	$ 15.69	$ 16.64	$ 16.76	$ 16.75	$ 18.09
Syndicated Facility [Member]
Capital Stock, Long-Term Debt, and Other Securities [Abstract]
Long Term Debt, Title [Text Block]	Syndicated Facility
OSI2 Citibank Facility [Member]
Capital Stock, Long-Term Debt, and Other Securities [Abstract]
Long Term Debt, Title [Text Block]	OSI2 Citibank Facility
2025 Notes [Member]
Capital Stock, Long-Term Debt, and Other Securities [Abstract]
Long Term Debt, Title [Text Block]	2025 Notes")
2027 Notes [Member]
Capital Stock, Long-Term Debt, and Other Securities [Abstract]
Long Term Debt, Title [Text Block]	2027 Notes
2029 Notes [Member]
Capital Stock, Long-Term Debt, and Other Securities [Abstract]
Long Term Debt, Title [Text Block]	2029 Notes