Ovarian cancer remains the most lethal gynaecological malignancy, and it exhibits substantial histologic and molecular heterogeneity. Traditional systemic therapies have produced only modest survival gains. Integration of immunohistochemical (IHC) and molecular diagnostics has shifted care toward biomarker-driven personalised treatment. This review presents current IHC approaches for histotype classification and the most important actionable protein markers (FR, HER2, TROP2, MMR proteins, and PD-L1), together with molecular testing for BRCA1/2, homologous recombination deficiency (HRD) status, and circulating tumour DNA (ctDNA) monitoring. These diagnostics now direct poly(ADP-ribose) polymerase inhibitors maintenance in HRD-positive disease and antibody-drug conjugates such as mirvetuximab soravtansine in FR-high platinum-resistant tumours. However, important limitations remain: intratumoral heterogeneity, variable assay performance, lack of standardisation, and unequal access to testing worldwide. We also discuss how best to combine multiple biomarkers and outline future directions, such as AI-assisted digital pathology, composite predictive models, and ctDNA-guided adaptive strategies, to refine patient selection and improve long-term outcomes.