Citius Oncology Inc美股招股说明书(2026-06-01版)

This prospectus supplement updates, amends and supplements the prospectus dated March 2, 2026 (as supplemented oramended from time to time, the “Prospectus”), which forms a part of the Post-Effective Amendment to the Registration Statement onForm S-1 (Registration No. 333-288656). Capitalized terms used in this prospectus supplement and not otherwise defined herein havethe meanings specified in the Prospectus. As disclosed in our Current Report on Form 8-K filed on May 6, 2026, the 6,818,182 Warrants registered herein wereexercised at a reduced exercised price of $0.90 per share pursuant to a warrant inducement agreement entered into by us and the holderand dated May 5, 2026. The net proceeds to the Company upon exercise of the Warrants was approximately $6.1 million. The 272,727Placement Agent’s Warrants remain outstanding. This prospectus supplement is being filed to update, amend and supplement the information included in the Prospectus withthe information contained in our Current Report on Form 8-K filed with the Securities and Exchange Commission on June 1, 2026 (the“Form 8-K”). Accordingly, we have attached the Form 8-Kand related exhibits to this prospectus supplement. This prospectus supplement is not complete without the Prospectus and is not complete without, and may not be delivered orutilized except in combination with, the Prospectus, including any amendments or supplements thereto. This prospectus supplementshould be read in conjunction with the Prospectus and if there is any inconsistency between the information in the Prospectus and thisprospectus supplement, you should rely on the information in this prospectus supplement. Our Common Stock is listed on the Nasdaq Capital Market under the symbol “CTOR.” The last reported closing price for ourCommon Stock on May 29, 2026, was $0.798 per share. Investing in our securities involves a high degree of risk, including the risk of losing your entire investment. See “RiskFactors” beginning on page 8 of the Prospectus to read about factors you should consider before investing in our securities. Neither the Securities and Exchange Commission nor any other regulatory body has approved or disapproved of thesesecurities or passed upon the accuracy or adequacy of this prospectus supplement. Any representation to the contrary is acriminal offense. The date of this prospectus supplement is June 1, 2026 UNITED STATESSECURITIES AND EXCHANGE COMMISSIONWashington, D.C. 20549 FORM 8-K CURRENT REPORTPursuant to Section 13 or 15(d) ofthe Securities Exchange Act of 1934 Date of Report (Date of earliest event reported)June 1, 2026 Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrantunder any of the following provisions: ☐Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)☐Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)☐Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))☐Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c)) Securities registered pursuant to Section 12(b) of the Act: Indicate by check mark whether the registrant is an emerging growth company as defined in Rule 405 of the Securities Act of 1933(§230.405 of this chapter) or Rule 12b-2 of the Securities Exchange Act of 1934 (§240.12b-2 of this chapter). Emerging growth company☒ If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period forcomplying with any new or revised financial accounting standards provided pursuant to Section 13(a) of the Exchange Act.☐ Item 8.01 Other Events. On June 1, 2026, Citius Oncology, Inc. issued a press release announcing the presentation of clinical data at the AnnualMeeting of the American Society of Clinical Oncology. A copy of the press release is furnished as Exhibit 99.1 to this Current Reporton Form 8-K. ExhibitNo.Description99.1Press Release, dated June 1, 2026.104Cover Page Interactive Data File (embedded within the Inline XBRL document). SIGNATURES Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed onits behalf by the undersigned hereunto duly authorized. CITIUS ONCOLOGY, INC. Date: June 1, 2026 Citius Oncology Highlights Phase 1 Data in an Investigator-Initiated Study of LYMPHIR®(denileukin diftitox-cxdl) in Combination with Pembrolizumab in Recurrent or Refractory Gynecologic Malignancies Investigator-initiated study data presented May 30, 2026, at the American Society ofClinical Oncology (ASCO) Annual Meeting demonstrated durable responses and manageable tolerability in heavily pre-treatedpatients 20.5 months of median progression-free survival observed among 48% of efficacy-evaluable patients achieving clinical benefit (10 of21) Responses were ob