Through comparison between the oxygen precipitation (OP) behaviors in heavily and lightly phosphorus (P)-doped Czochralski silicon (CZ-Si) crystals subjected to low-high two-step anneal of 600, 650, or 750 °C/8 h+1050 °C/16 h, we have found that in heavily P-doped CZ-Si, OP is much stronger in the case with the nucleation anneal at 600 or 650 °C while it is to some extent suppressed in the case with the nucleation anneal at 750 °C in contrast to lightly doped CZ-Si where nucleation is enhanced at 750 °C. Transmission electron microscopy investigation reveals that silicon phosphide precipitates of face-centered-cubic SiP form during the nucleation anneal at temperatures 650 °C and below. The SiP precipitates act as the heterogeneous nuclei for OP during the subsequent high temperature anneal while the oxygen precipitate nuclei containing certain amounts of P atoms generate during the nucleation anneal at 750 °C. They are further coarsened to be larger oxygen precipitates during the subsequent high temperature anneal. Of significance is that the oxygen precipitate nucleation mechanism operating at certain temperatures for heavily P-doped CZ-Si is found to be completely different from that of lightly P-doped CZ-Si. We believe that the present work provides a further insight into OP in CZ-Si.