Supplemental Prospectus (Form 424B3)

Filed Pursuant to Rule 424(b)(3)

File No. 333-237929

FLAT ROCK OPPORTUNITY FUND

SUPPLEMENT NO. 1, DATED SEPTEMBER 15, 2026

TO THE PROSPECTUS AND STATEMENT OF ADDITIONAL INFORMATION

DATED JUNE 29, 2026

This document supplements, and should be read in conjunction with, the Fund's prospectus and statement of additional information, each dated June 29, 2026, relating to the Fund's offering of unlimited common shares of beneficial interest ("Shares"). Terms used and not otherwise defined in this Supplement No. 1 shall have the same meanings as set forth in the Fund's prospectus and/or statement of additional information.

The purpose of this supplement is to announce the following:

Management Changes

Effective September 15, 2026, Mr. Ripp resigned his position as Chief Financial Officer of the Fund. Mr. Ripp's resignation was not due to any disagreements with management of the Fund or the operations, policies or practices of the Fund. Mr. Ripp will continue his employment with Flat Rock Global LLC. On September 15, 2026, the Board of Trustees of the Fund (the "Board") appointed Mr. Stewart to serve as Chief Financial Officer and Treasurer of the Fund. All references to Mr. Ripp as Chief Financial Officer in the prospectus and statement of additional information should be disregarded. As of September 15, 2026, Mr. Stewart did not own any shares of the Fund.

The table entitled "Officers Who Are Not Trustees" under the section "Management of the Fund" in the statement of additional information is replaced in its entirety, as follows:

Officers Who Are Not Trustees

Information regarding our officers who are not trustees is as follows. The address for each officer is c/o Flat Rock Opportunity Fund, 680 S. Cache Street, Suite 100, P.O. Box 7403, Jackson, WY 83001, unless otherwise noted.

(1) Officers are typically elected every year, unless an officer earlier retires, resigns or is removed from office.

The biography of Ryan Ripp is removed from the section "Biographical Information - Officers Who are not Trustees" in the statement of additional information, and the following is added:

Michael Stewart. Mr. Stewart has served as Chief Financial Officer and Treasurer of the Fund since September 2026. Mr. Stewart has been a Vice President and Fund Controller at Flat Rock Global since January 2025. Prior to joining Flat Rock Global, Mr. Stewart spent five years at Ernst & Young in their Wealth and Asset Management assurance practice, focusing on registered and private fund audits. Mr. Stewart graduated from Boston University in 2019 with a Bachelor of Science in Business Administration, with concentrations in Accounting and Entrepreneurship. Mr. Stewart is a licensed CPA.