Ten years of continuous current meter recordings near the core of the Peru-Chile Undercurrent (PCU) over the continental slope at 30°S off Chile, local and equatorial wind stress data, and monthly oceanographic sections off Valparaíso (33°S) are used to study the variability of the PCU and the Oxygen Minimum Zone (OMZ) off Chile. The physics that governs the PCU variability differs for intraseasonal periods and for longer periods: Coastal trapped waves dominate the intraseasonal band whereas Rossby waves dominate for periods greater than about 120 days. Semiannual and annual period fluctuations can largely be explained by Rossby waves forced by equatorial Kelvin waves and local winds. Seasonal variations in the PCU are strongly modulated over El Niño/La Niña cycles. During warm El Niño events, the PCU undergoes strong, semiannual variability whereas during La Niña events, annual variability dominates. These results together with oceanographic sections off Valparaíso suggest that Rossby waves provide a physical mechanism to explain a significant fraction of seasonal variability observed in the OMZ. During the downwelling (upwelling) phase of these waves the poleward (equatoward) flow is stronger (90° out of phase with respect to sea level), the isotherms are deeper (shallower), the OMZ thickness increases (decreases), the mean concentration of salinity and silicate increases (decreases) and oxygen concentration and nitrate deficit decrease (increase) in the OMZ.