This systematic review and meta-analysis evaluated the prognostic and therapeutic implications of BRCA1 expression, assessed via immunohistochemistry (IHC), in ovarian cancer patients. A comprehensive search of PubMed and Epistemonikos databases yielded 18 studies (n = 2,830 patients) for quantitative analysis. The pooled prevalence of BRCA1 deficiency was 49.51% (95% CI: 40.27–58.78), with significant heterogeneity across studies (I² = 67.59%, p < 0.001). Random-effects modeling confirmed a robust association between BRCA1 negativity and ovarian cancer prognosis (z = 10.5, p < 0.001), with effect sizes ranging widely (9.00–84.40%), reflecting variability in patient populations and detection methods. BRCA1-deficient tumors demonstrated poorer responses to platinum-based therapies but heightened sensitivity to PARP inhibitors. These findings underscore BRCA1 IHC as a critical biomarker for prognostic stratification and personalized treatment strategies. Standardization of BRCA1 assessment protocols is recommended to enhance clinical applicability.