Oxygen precipitation in Czochralski-grown silicon intentionally contaminated with Cu or Fe is investigated by means of Fourier-transform infrared spectroscopy, transmission electron microscopy, and the electron-beam-induced-current technique. It is found that oxygen precipitation is not influenced by Cu impurities, but is enhanced significantly by Fe impurities even if the concentration of Fe is much lower than that of Cu in Si. Cu forms precipitate colonies of a low density and a large size, and does not react with minute oxygen precipitates existing in as-grown specimens. On the contrary, Fe precipitates on them and forms Fe decorated silica particles of a very high density, which serve as the efficient sites for the nucleation of oxygen precipitates.