Repeated deep-frying is one of the most thermally intensive uses of edible oils in Nigerian cooking and a major cause of lipid oxidation and toxic compound formation. This study comparatively evaluated the thermal oxidative stability of refined palm oil (PO), palm kernel oil (PKO), and groundnut oil (GNO) brands sold in Port Harcourt, Rivers State, Nigeria under simulated deep-frying conditions (180 ± 5°C; 0, 2, 4, 6, and 8 cumulative frying hours). Six commercial brands were assessed (two per oil type). Key quality indicators included total polar compounds (TPC), determined by column chromatography as the primary frying oil degradation marker; polymer content (PC); and secondary indices such as peroxide value (PV), p-anisidine value (p-AV), total oxidation value (TOTOX), free fatty acid (FFA), viscosity, colour (Gardner scale), and specific extinction coefficients (K232, K270). Results showed that refined PO exhibited the highest thermal stability, with TPC values of 22.4–24.6% after 8 hours, remaining below the 25% discard limit. PKO showed moderate stability (TPC: 18.8–21.4% at 8 hours), reflecting its saturated fatty acid composition. GNO demonstrated the lowest stability, with TPC values of 28.6–31.4% at 8 hours, exceeding the discard threshold between 6 and 8 hours. Polymer formation followed the order GNO > PO > PKO across all frying durations. Viscosity increased 2.1- to 3.4-fold after 8 hours, with GNO showing the highest increase. This study provides the first brand-specific comparative dataset on frying stability of these oils in Rivers State and supports evidence-based guidelines for frying oil reuse and replacement in both domestic and industrial Nigerian food applications.