Density spectra of grown-in oxide precipitate nuclei were measured along the radius of a silicon wafer with a stacking fault ring. A first-order approximation model allows us to explain the experimental observations. By fitting growth rate curves to the defect density spectra, the residual point defect supersaturation present during formation of oxide precipitate nuclei can be estimated. We show that the ring region is the region where the vacancy supersaturation remains subcritical and no vacancy agglomeration occurs, resulting in the highest residual vacancy supersaturation, and finally leading to the largest grown-in oxide precipitate nuclei. These nuclei first reach the critical size for stacking fault formation during subsequent thermal treatments.