Borrowings

12 Months Ended

Sep. 30, 2025

Debt Disclosure [Abstract]

Borrowings

Borrowings

Syndicated Facility

On November 30, 2017, the Company entered into a senior secured revolving credit facility (as amended and restated, the “Syndicated Facility”) pursuant to a Senior Secured Revolving Credit Agreement with the lenders party thereto, ING Capital LLC, as administrative agent, ING Capital LLC, JPMorgan Chase Bank, N.A., BofA Securities, Inc. and Wells Fargo Securities, LLC, as joint lead arrangers and joint bookrunners, and JPMorgan Chase Bank, N.A. and Bank of America, N.A., as syndication agents. The Syndicated Facility provides that the Company may use the proceeds of the loans and issuances of letters of credit under the Syndicated Facility for general corporate purposes, including acquiring and funding leveraged loans, mezzanine loans, high-yield securities, convertible securities, preferred stock, common stock and other investments. The Syndicated Facility further allows the Company to request letters of credit from ING Capital LLC, as the issuing bank.

As of September 30, 2025, the size of the Syndicated Facility was $1.16 billion. In addition, pursuant to an "accordion" feature, the Company may increase the size of the facility to up to the greater of $1.50 billion and the Company's net worth, as defined in the facility, under certain circumstances.

As of September 30, 2025, (i) the period during which the Company may make drawings will expire on April 8, 2029 and the maturity date is April 8, 2030 and (ii) the interest rate margin for (a) SOFR loans (which may be 1- or 3-month at the

Company’s option) was 1.875% plus a SOFR adjustment equal to 0.10% and (b) alternate base rate loans was 0.875% plus a SOFR adjustment equal to 0.10%; provided that, if at any time the Borrowing Base (as defined in the Syndicated Facility) is greater than 1.60 times the Combined Debt Amount (as defined in the Syndicated Facility), the interest rate margin with respect to (a) SOFR loans will be 1.75% plus a SOFR adjustment equal to 0.10% and (b) alternate base rate loans will be 0.75% plus a SOFR adjustment equal to 0.10%.

The Syndicated Facility is secured by substantially all of the Company’s assets (excluding, among other things, investments held in and by certain subsidiaries of the Company (including OSI 2 Senior Lending SPV, LLC, or “OSI 2 SPV”) or investments in certain portfolio companies of the Company) and guaranteed by certain subsidiaries of the Company.

The Syndicated Facility requires the Company to, among other things, (i) make representations and warranties regarding the collateral as well as each of the Company’s portfolio companies’ businesses, (ii) agree to certain indemnification obligations, and (iii) comply with various affirmative and negative covenants, reporting requirements and other customary requirements for similar revolving credit facilities, including covenants related to: (A) limitations on the incurrence of additional indebtedness and liens, (B) limitations on certain investments, (C) limitations on certain asset transfers and restricted payments, (D) maintaining a certain minimum stockholders’ equity, (E) maintaining a ratio of total assets (less total liabilities) to total indebtedness, of the Company and its subsidiaries (subject to certain exceptions), of not less than 1.50 to 1.00, (F) maintaining a ratio of consolidated EBITDA to consolidated interest expense, of the Company and its subsidiaries (subject to certain exceptions), of not less than 2.25 to 1.00, (G) maintaining a minimum liquidity and net worth, and (H) limitations on the creation or existence of agreements that prohibit liens on certain properties of the Company and certain of its subsidiaries. The Syndicated Facility also includes usual and customary default provisions such as the failure to make timely payments under the facility, the occurrence of a change in control, and the failure by the Company to materially perform under the agreements governing the facility, which, if not complied with, could accelerate repayment under the facility. As of September 30, 2025, the Company was in compliance with all financial covenants under the Syndicated Facility. In addition to the asset coverage ratio described above, borrowings under the Syndicated Facility (and the incurrence of certain other permitted debt) are subject to compliance with a borrowing base that will apply different advance rates to different types of assets in the Company’s portfolio. Each loan or letter of credit originated or assumed under the Syndicated Facility is subject to the satisfaction of certain conditions. In connection with the April 8, 2025 amendment of the Syndicated Facility, the Company accelerated $0.7 million of deferred financing costs into interest expense during the year ended September 30, 2025.

As of September 30, 2025 and September 30, 2024, the Company had $545.0 million and $430.0 million of borrowings outstanding under the Syndicated Facility, which had a fair value of $545.0 million and $430.0 million, respectively. The Company's borrowings under the Syndicated Facility bore interest at a weighted average interest rate of 6.467%, 7.443% and 6.792% for the years ended September 30, 2025, 2024 and 2023, respectively. For the years ended September 30, 2025, 2024 and 2023, the Company recorded interest expense (inclusive of fees) of $36.8 million, $39.4 million and $50.0 million, respectively, related to the Syndicated Facility.

OSI2 Citibank Facility

On January 23, 2023, as a result of the consummation of the OSI2 Merger, the Company became party to a revolving credit facility (as amended and/or restated from time to time, the “OSI2 Citibank Facility”) with OSI 2 SPV, the Company’s wholly-owned and consolidated subsidiary, as the borrower, the Company, as collateral manager, each of the lenders from time to time party thereto, Citibank, N.A., as administrative agent, and Deutsche Bank Trust Company Americas, as collateral agent. On May 14, 2025, the Company repaid all outstanding borrowings under the OSI2 Citibank Facility, following which the OSI2 Citibank Facility was terminated. Obligations under the OSI2 Citibank Facility would have otherwise matured on January 26, 2029.

In connection with the termination of the OSI2 Citibank Facility, the Company accelerated $3.1 million of deferred financing costs into interest expense during the year ended September 30, 2025.

As of September 30, 2024, the Company had $280.0 million outstanding under the OSI2 Citibank Facility, which had a fair value of $280.0 million. The Company’s borrowings under the OSI2 Citibank Facility bore interest at a weighted average interest rate of 6.741%, 7.756% and 7.666% for the years ended September 30, 2025 and 2024 and the period from January 23, 2023 to September 2023, respectively. For the years ended September 30, 2025 and 2024, the Company recorded interest expense (inclusive of fees) of $14.4 million and $23.8 million, respectively, related to the OSI2 Citibank Facility. For the period from January 23, 2023 to September 2023, the Company recorded interest expense (inclusive of fees) of $14.6 million related to the OSI2 Citibank Facility.

2025 Notes

On February 25, 2020, the Company issued $300.0 million in aggregate principal amount of the 2025 Notes for net proceeds of $293.8 million after deducting OID of $2.5 million, underwriting commissions and discounts of $3.0 million and offering costs of $0.7 million. The OID on the 2025 Notes was amortized based on the effective interest method over the term of the 2025 Notes.

Interest on the 2025 Notes was paid semi-annually on February 25 and August 25 at a rate of 3.500% per annum. The 2025 Notes matured on February 25, 2025.

2027 Notes

On May 18, 2021, the Company issued $350.0 million in aggregate principal amount of the 2027 Notes for net proceeds of $344.8 million after deducting OID of $1.0 million, underwriting commissions and discounts of $3.5 million and offering costs of $0.7 million. The OID on the 2027 Notes is amortized based on the effective interest method over the term of the 2027 Notes.

The 2027 Notes were issued pursuant to an indenture, dated April 30, 2012, as supplemented by the sixth supplemental indenture, dated May 18, 2021 (collectively, the "2027 Notes Indenture"), between the Company and the Trustee. The 2027 Notes are the Company's general unsecured obligations that rank senior in right of payment to all of the Company's existing and future indebtedness that is expressly subordinated in right of payment to the 2027 Notes. The 2027 Notes rank equally in right of payment with all of the Company's existing and future liabilities that are not so subordinated. The 2027 Notes effectively rank junior to any of the Company's secured indebtedness (including unsecured indebtedness that the Company later secures) to the extent of the value of the assets securing such indebtedness. The 2027 Notes rank structurally junior to all existing and future indebtedness (including trade payables) incurred by the Company's subsidiaries, financing vehicles or similar facilities.

Interest on the 2027 Notes is paid semi-annually on January 15 and July 15 at a rate of 2.700% per annum. The 2027 Notes mature on January 15, 2027 and may be redeemed in whole or in part at any time or from time to time at the Company's option prior to maturity at par plus a “make-whole” premium, if applicable. In addition, holders of the 2027 Notes can require the Company to repurchase the 2027 Notes at 100% of their principal amount upon the occurrence of certain change of control events as described in the 2027 Notes Indenture. The 2027 Notes were issued in minimum denominations of $2,000 and integral multiples of $1,000 in excess thereof. During the year ended September 30, 2025, the Company did not repurchase any of the 2027 Notes in the open market.

The 2027 Notes Indenture contains certain covenants, including covenants requiring the Company's compliance with the asset coverage requirements set forth in Section 18(a)(1)(A) as modified by Section 61(a)(1) and (2) of the Investment Company Act or any successor provisions (but giving effect to any exemptive relief granted to the Company by the Securities and Exchange Commission (the "SEC"), as well as covenants requiring the Company to provide financial information to the holders of the 2027 Notes and the Trustee if the Company ceases to be subject to the reporting requirements of the Securities Exchange Act of 1934, as amended (the "Exchange Act"). These covenants are subject to limitations and exceptions that are described in the 2027 Notes Indenture.

In connection with the 2027 Notes, the Company entered into an interest rate swap to more closely align the interest rates of its liabilities with its investment portfolio, which consists of predominately floating rate loans. Under the interest rate swap agreement, the Company receives a fixed interest rate of 2.700% and pays a floating interest rate of the three-month SOFR plus 1.658% plus a SOFR adjustment of 0.26161% on a notional amount of $350.0 million. The Company designated the interest rate swap as the hedging instrument in an effective hedge accounting relationship. See Note 12 for more information regarding the interest rate swap.

2029 Notes

On August 15, 2023, the Company issued $300.0 million in aggregate principal amount of the 2029 Notes for net proceeds of $292.9 million after deducting OID of $3.5 million, underwriting commissions and discounts of $3.0 million and offering costs of $0.6 million. The OID on the 2029 Notes is amortized based on the effective interest method over the term of the 2029 Notes.

The 2029 Notes were issued pursuant to an indenture, dated April 30, 2012, as supplemented by the seventh supplemental indenture, dated August 15, 2023 (collectively, the "2029 Notes Indenture"), between the Company and the Trustee. The 2029 Notes are the Company's general unsecured obligations that rank senior in right of payment to all of the Company's existing and future indebtedness that is expressly subordinated in right of payment to the 2029 Notes. The 2029 Notes rank equally in right

of payment with all of the Company's existing and future liabilities that are not so subordinated. The 2029 Notes effectively rank junior to any of the Company's secured indebtedness (including unsecured indebtedness that the Company later secures) to the extent of the value of the assets securing such indebtedness. The 2029 Notes rank structurally junior to all existing and future indebtedness (including trade payables) incurred by the Company's subsidiaries, financing vehicles or similar facilities.

Interest on the 2029 Notes is paid semi-annually on February 15 and August 15 at a rate of 7.100% per annum. The 2029 Notes mature on February 15, 2029 and may be redeemed in whole or in part at any time or from time to time at the Company's option prior to maturity at par plus a “make-whole” premium, if applicable. In addition, holders of the 2029 Notes can require the Company to repurchase the 2029 Notes at 100% of their principal amount upon the occurrence of certain change of control events as described in the 2029 Notes Indenture. The 2029 Notes were issued in minimum denominations of $2,000 and integral multiples of $1,000 in excess thereof. During the year ended September 30, 2025, the Company did not repurchase any of the 2029 Notes in the open market.

The 2029 Notes Indenture contains certain covenants, including covenants requiring the Company's compliance with the asset coverage requirements set forth in Section 18(a)(1)(A) as modified by Section 61(a)(1) and (2) of the Investment Company Act or any successor provisions (but giving effect to any exemptive relief granted to the Company by the SEC), as well as covenants requiring the Company to provide financial information to the holders of the 2029 Notes and the Trustee if the Company ceases to be subject to the reporting requirements of the Exchange Act. These covenants are subject to limitations and exceptions that are described in the 2029 Notes Indenture.

In connection with the 2029 Notes, the Company entered into an interest rate swap to more closely align the interest rates of its liabilities with its investment portfolio, which consists of predominately floating rate loans. Under the interest rate swap agreement, the Company receives a fixed interest rate of 7.100% and pays a floating interest rate of the three-month SOFR plus 3.1255% on a notional amount of $300.0 million. The Company designated the interest rate swap as the hedging instrument in an effective hedge accounting relationship. See Note 12 for more information regarding the interest rate swap.

2030 Notes

On February 27, 2025, the Company issued $300.0 million in aggregate principal amount of the 2030 Notes for net proceeds of $296.3 million after deducting OID of less than $0.1 million, underwriting commissions and discounts of $3.0 million and offering costs of $0.7 million. The OID on the 2030 Notes is amortized based on the effective interest method over the term of the 2030 Notes.

The 2030 Notes were issued pursuant to an indenture, dated April 30, 2012, as supplemented by the eighth supplemental indenture, dated February 27, 2025 (collectively, the "2030 Notes Indenture"), between the Company and the Trustee. The 2030 Notes are the Company's general unsecured obligations that rank senior in right of payment to all of the Company's existing and future indebtedness that is expressly subordinated in right of payment to the 2030 Notes. The 2030 Notes rank equally in right of payment with all of the Company's existing and future liabilities that are not so subordinated. The 2030 Notes effectively rank junior to any of the Company's secured indebtedness (including unsecured indebtedness that the Company later secures) to the extent of the value of the assets securing such indebtedness. The 2030 Notes rank structurally junior to all existing and future indebtedness (including trade payables) incurred by the Company's subsidiaries, financing vehicles or similar facilities.

Interest on the 2030 Notes is paid semi-annually on February 27 and August 27 at a rate of 6.340% per annum. The 2030 Notes mature on February 27, 2030 and may be redeemed in whole or in part at any time or from time to time at the Company's option prior to maturity at par plus a “make-whole” premium, if applicable. In addition, holders of the 2030 Notes can require the Company to repurchase the 2030 Notes at 100% of their principal amount upon the occurrence of certain change of control events as described in the 2030 Notes Indenture. The 2030 Notes were issued in minimum denominations of $2,000 and integral multiples of $1,000 in excess thereof. During the year ended September 30, 2025, the Company did not repurchase any of the 2030 Notes in the open market.

The 2030 Notes Indenture contains certain covenants, including covenants requiring the Company's compliance with the asset coverage requirements set forth in Section 18(a)(1)(A) as modified by Section 61(a)(1) and (2) of the Investment Company Act or any successor provisions (but giving effect to any exemptive relief granted to the Company by the SEC), as well as covenants requiring the Company to provide financial information to the holders of the 2030 Notes and the Trustee if the Company ceases to be subject to the reporting requirements of the Exchange Act. These covenants are subject to limitations and exceptions that are described in the 2030 Notes Indenture.

In connection with the 2030 Notes, the Company entered into an interest rate swap to more closely align the interest rates of its liabilities with its investment portfolio, which consists of predominately floating rate loans. Under the interest rate swap

agreement, the Company receives a fixed interest rate of 6.340% and pays a floating interest rate of the three-month SOFR plus 2.192% on a notional amount of $300.0 million. The Company designated the interest rate swap as the hedging instrument in an effective hedge accounting relationship. See Note 12 for more information regarding the interest rate swap.

The below table presents the components of the carrying value of the 2025 Notes, the 2027 Notes, the 2029 Notes and the 2030 Notes as of September 30, 2025 and September 30, 2024:

	As of September 30, 2025						As of September 30, 2024
($ in millions)	2027 Notes		2029 Notes		2030 Notes		2025 Notes		2027 Notes		2029 Notes
Principal	$	350.0	$	300.0	$	300.0	$	300.0	$	350.0	$	300.0
Unamortized financing costs	(1.0)		(2.2)		(3.3)		(0.3)		(1.8)		(2.9)
Unaccreted discount	(0.2)		(2.1)		—		(0.2)		(0.4)		(2.7)
Interest rate swap fair value adjustment	(12.2)		4.8		8.1		—		(20.2)		7.2
Net carrying value	$	336.6	$	300.5	$	304.8	$	299.5	$	327.6	$	301.6
Fair Value	$	339.8	$	314.5	$	301.1	$	298.1	$	327.7	$	312.3

The below table presents the components of interest and other debt expenses related to the 2025 Notes, the 2027 Notes, the 2029 Notes and the 2030 Notes for the year ended September 30, 2025:

($ in millions)	2025 Notes			2027 Notes			2029 Notes			2030 Notes
Coupon interest	$	4.2		$	9.4		$	21.3		$	11.3
Amortization of financing costs and discount	0.5			0.9			1.3			0.4
Effect of interest rate swap	—			13.1			1.8			0.4
Total interest expense	$	4.7		$	23.4		$	24.4		$	12.1
Coupon interest rate (net of effect of interest rate swaps)	3.500		%	6.353		%	7.585		%	6.491		%

The below table presents the components of interest and other debt expenses related to the 2025 Notes, the 2027 Notes and the 2029 Notes for the year ended September 30, 2024:

($ in millions)	2025 Notes			2027 Notes			2029 Notes
Coupon interest	$	10.5		$	9.5		$	21.3
Amortization of financing costs and discount	1.3			0.9			1.3
Effect of interest rate swap	—			16.3			4.4
Total interest expense	$	11.8		$	26.7		$	27.0
Coupon interest rate (net of effect of interest rate swaps)	3.500		%	7.259		%	8.437		%

The below table presents the components of interest and other debt expenses related to the 2025 Notes, the 2027 Notes and the 2029 Notes for the year ended September 30, 2023:

($ in millions)	2025 Notes			2027 Notes			2029 Notes
Coupon interest	$	10.5		$	9.5		$	2.7
Amortization of financing costs and discount	1.3			0.9			0.2
Effect of interest rate swap	—			13.4			0.6
Total interest expense	$	11.8		$	23.8		$	3.5
Coupon interest rate (net of effect of interest rate swaps)	3.500		%	6.539		%	8.490		%