Oxygen precipitation in Czochralski silicon heated in the range 400–1050°C is reviewed. For T≥525° C, Si02 particles form at the normal diffusion rate and there is generation of self-interstitials. At the lower temperatures, the existence of the interfacial energy causes an apparent increase in the solid solubility as the agglomerates become very small: at 525° C they contain only an estimated 20–50 atoms. A critical analysis s then presented of possible oxygen aggregation reactions at even lower temperatures when thermal donors are generated. It is not yet possible to reconcile the kinetics of these two processes, even if self-interstitials and/or vacancy reactions are included. There is no evidence for enhanced diffusion of isolated oxygen atoms except as a transient process occurring during the relaxation of stress-induced dichroism. Oxygen aggregation at 450 ° C appears to be limited by the formation of dimers with an activation energy of 2.5eV, while thermal donors form with an activation energy of 1.7eV.