This paper reports some phenomena of the nucleation of oxygen precipitates in silicon, and proposes a nucleation model consistent with these observations. In addition to the principal tool of infrared absorption for the measurement of dissolved oxygen concentrations, the distribution of the dissolved and the precipitated oxygen was characterized by a method combining thermal conversion and spreading resistance profiling, a method combining thermal conversion and copper plating, and a selective etching. Heat treatments were carried out in argon ambients at temperatures from 800 to 1150°C, for time up to 96 h. In many cases samples from seed‐end segments of Czochralski crystals, with uniform initial oxygen distribution, exhibited alternating zones of fast and slow precipitation. Both the low density and the clustering aspects of the precipitates in the slow precipitation zones are reminiscent of precipitates formed in oxidizing ambients I previously reported. The density of precipitates was observed to increase with annealing time, and for sufficiently long time, with the decrease of annealing temperature. In sequential annealing processes, a low‐temperature preanneal was found to accelerate the precipitation in a subsequent higher‐temperature anneal. These two phenomena are evidence of a homogeneous nucleation process. It was also observed that a high‐temperature preanneal tended to irreversibly retard the precipitation in a subsequent lower‐temperature anneal. Both this phenomenon and the zonal variation of precipitation rates are not expected from the conventional concept of homogeneous nucleation, which involves only the agglomeration of solute atoms. However, both can be explained by a modified homogeneous nucleation model that involves both the oxygen atoms and certain active centers, most probably small vacancy complexes. These small vacancy complexes are often incorporated in striae during crystal growth, and are susceptible both to annihilation by oxidation induced excess self‐interstitials, and to breakup by high‐temperature preanneals. A surface proximity effect is also reported, in which oxygen in a surface zone immediately below the out‐diffusion zone precipitated out faster than in bulk.