In this paper we have investigated the role of the Er–O interaction on the photoluminescence properties of Er-doped crystalline silicon. We demonstrate that the strong Er–O interaction leads to an O redistribution after thermal treatment. In particular, in the absence of Er, O out-diffuses from the Si surface and a depletion of O in the first micron below the surface is observed. In the presence of Er, the rare earth acts as a getter for O and therefore there is an O redistribution as a result of the balance between O out-diffusion and O trapping by the Er. This implies that the O concentration that remains in the Si crystal changes after thermal annealing and it is largely determined by the amount of Er present in the sample. These data will be presented and the implication on the Er optical activity discussed.