Background/Objectives: Ovarian cancer is the most lethal gynecologic malignancy, largely due to late diagnosis and the high prevalence of malignant ascites, a hallmark of advanced disease that is difficult to control and contributes to immune suppression and treatment failure. Despite advances in standard care, durable responses are rare. This study investigates a novel immunotherapeutic strategy designed to overcome the suppressed peritoneal microenvironment using an oncolytic vaccinia virus engineered to express a decoy-resistant IL-18 mutein. Methods: We generated a vaccinia virus (vvDD-nsmDR-18) expressing a non-secreted, mature, decoy-resistant IL-18. Viral expression was validated via RT-qPCR and fluorescence microscopy, while cytotoxicity was confirmed using CCK-8 assays. The antitumor efficacy of vvDD-nsmDR-18 was evaluated in the aggressive murine ID8a ovarian cancer model. The underlying mechanisms of action were investigated using flow cytometry and transcriptional profiling. Results: Treatment with vvDD-nsmDR-18 significantly prolonged survival and was associated with reduced abdominal distension consistent with decreased ascites burden. Immune analyses indicated enhanced T cell activation across multiple anatomical compartments, including tumors, peritoneal cavity, and spleens, the latter recently suggested to serve as a reservoir for tumor-reactive T cells. This systemic activation was characterized by increased IFN-γ and perforin expression. In addition, vvDD-nsmDR-18 treatment was associated with expansion of CD39⁺CD103⁺CD8⁺ tumor-reactive T cells and a shift toward a lower PD-1 expression phenotype within this population. Conclusions: These findings demonstrate that nsmDR-18-expressing oncolytic viruses can remodel the immunosuppressive landscape of advanced ovarian cancer, suggesting this approach is a promising candidate for further clinical development.