Previously reported measurements of anomalously high rates of oxygen out-diffusion in Czochralski silicon at low temperatures (T≤450°C) are confirmed. The surface concentration is shown to decrease with increasing time while the depth to which the concentration is depleted remains constant. Exposure to a hydrogen plasma under conditions known to catalyse the diffusion of isolated oxygen atoms causes an increased rate of decrease of the surface concentration without significantly affecting the depth to which the concentration is depleted. The evolution of the out-diffusion profiles cannot be explained by a catalytic mechanism operating on the isolated oxygen atoms. A slow conversion of Oi to a complex containing oxygen which can diffuse rapidly over long distances before being trapped either on the surface or in the bulk of the sample can account for both out-diffusion and simultaneous loss of [Oi] in the bulk. The conversion rate is enhanced by exposure to a hydrogen plasma, indicating that it is controlled by the diffusion rate of isolated atoms.