Background <p>Ovarian carcinosarcoma, also known as malignant mixed Müllerian tumor (MMMT), is a rare and highly aggressive gynecologic malignancy characterized by the presence of both epithelial and mesenchymal components. It predominantly affects postmenopausal women, and its occurrence in premenopausal patients is uncommon. Due to its rarity, optimal management strategies remain poorly defined.</p> Case presentation <p>We report the case of a 33-year-old premenopausal Han Chinese woman diagnosed with ovarian carcinosarcoma who underwent complete cytoreductive surgery followed by platinum-based chemotherapy. Germline BRCA1/2 mutation testing and homologous recombination deficiency (HRD) assessment were performed and yielded negative results. Following a favorable response to chemotherapy, maintenance therapy with Olaparib was initiated as an individualized, off-label approach after multidisciplinary team discussion and with full patient-informed consent. Sustained disease control was observed during a 46-month follow-up period, without significant treatment-related adverse events.</p> Conclusion <p>This case highlights the importance of accurate diagnosis and optimal surgical and systemic management in ovarian carcinosarcoma. While maintenance therapy with Olaparib was used in an individualized, off-label setting despite negative BRCA and HRD testing, this approach should not be generalized. Further studies are needed to better define the role of targeted therapies in this rare and aggressive tumor subtype.</p>