The diffusion coefficient of oxygen in heavily antimony doped Czochralski Si was measured in the temperature range 950–1100 °C by using secondary ion mass spectroscopy (SIMS). The diffusion coefficient, obtained from SIMS oxygen concentration profiles in samples submitted to out diffusion, shows no dependence on antimony concentration. The combined data give an activation energy of 2.68 eV, which is in good agreement with published results.