Motion of the Earth's pole of rotation relative to its crust, commonly referred to as polar motion, can be excited by a variety of geophysical mechanisms(1), In particular, changes in atmospheric wind and mass fields have been linked to polar motion over a wide range of timescales, but substantial discrepancies remain between the atmospheric and geodetic observations(1-4). Here we present results from a nearly global ocean model which indicate that oceanic circulation and mass-field variability play important roles in the excitation of seasonal to fortnightly polar motion. The joint oceanic and atmospheric excitation provides a better agreement with the observed polar motion than atmospheric excitation alone. Geodetic measurements may therefore be used to provide a global consistency check on the quality oi simulated large-scale oceanic fields.