UNITED STATES SECURITIES AND EXCHANGE COMMISSION Washington, DC 20549 Form 10-K (Mark One) þ ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 For the fiscal year ended September 30, 2023 OR o TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE ACT OF 1934 COMMISSION FILE NUMBER: 1-33901 Oaktree Specialty Lending Corporation (EXACT NAME OF REGISTRANT AS SPECIFIED IN ITS CHARTER) Delaware 26-1219283 (State or jurisdiction of (I.R.S. Employer incorpo ...