<p>This study investigates the influence of the Quasi-BiWeekly Oscillation (QBWO) on the onset, monsoon breaks, and extreme rainfall events in the Philippines during the southwest monsoon season from 1980 to 2019. Findings show that the occurrence of extreme rainfall events over the Philippines follows the northward to northwestward movement of QBWO-related convective anomalies. These events initially appear in Mindanao and Visayas (Phase 5), then shift toward Luzon (Phases 7–8), and may persist along Luzon’s western coast up to Phase 2. The contribution of tropical cyclones (TCs), low-pressure systems (LPSs), and other disturbances to extreme rainfall was also assessed. Although their percentage contribution to the total extreme rainfall varies substantially across stations, their impacts are more apparent in Phase 6, coinciding with northward/northwestward propagation of the convective anomalies of the QBWO towards the Philippines. Monsoon breaks, which is defined as the period when the average rainfall across the western coastal stations in Luzon Island decreases below 5&#xa0;mm day^-1 for at least three consecutive days, also show distinct QBWO-phase preferences such that short to medium breaks are more frequent in Phases 3–5, coinciding with the movement of the anticyclonic anomaly of the QBWO from the South China Sea toward East Asia in these phases. In contrast, longer breaks occur more frequently in Phases 4–6 and may persist until Phase 7, when the northeasterlies at the northern flank of the cyclonic circulation anomaly of the QBWO weaken the southwest monsoon. The southwest monsoon onset in the Philippines typically begins during Phase 1 and Phases 6–8. Notably, about 82% of monsoon onset events occurred during these phases when the QBWO amplitude exceeded 0.5, suggesting that a real-time QBWO index could aid in monitoring the monsoon onset.</p>