The climatological annual cycle of oxygen content anomaly of the 0--100 m depth layer for major ocean basins is described using Fourier analysis of objectively analyzed monthly O2 anomaly values on a 1¿ grid (70¿S--70¿N). The largest seasonal changes in O2 content anomaly occur in the extra-tropics in the 30¿ to 60¿ latitude belt of each hemisphere. The magnitude of the global cycle is dominated by the Pacific Ocean. The annual and semi-annual harmonics account for most (>90%) of the annual cycle of the monthly O2 content anomaly. The magnitude of the annual harmonic is largest in the upper 75 m layer except in the tropics and high latitudes. The annual harmonic accounts for >90% of the variance of the zonally integrated monthly O2 content anomaly.