Transmission electron microscope studies of carbon‐doped Czochralski silicon, when combined with previous infrared data on the same specimens, reveal a double peak in the carbon‐sited oxygen‐cluster size distribution after 64 h at 750 °C. The first peak, which represents most of the carbon and oxygen in the specimen, is comprised of clusters with an average of two oxygens per carbon atom. These clusters can survive 64 h at 1000 °C although they are not created by such an anneal, suggesting that carbon atoms have difficulty trapping a first oxygen atom at 1000 °C. The second peak in the distribution near 104oxygen atoms in size is populated with regular {111}‐octahedral precipitates having large dilatational strain fields. The two peaks in the size distribution, and their dependences on heat treatment, indicate roles for both seeding (creation of metastable clusters below critical size) and nucleation (achievement of energetic stability) in the formation of carbon‐sited precipitates. The observations confirm a trend toward octahedral precipitate morphologies in carbon‐doped specimens. However, the trend may result not from site differences but from effects of carbon or point defects on strain energy during precritical cluster growth. Finally, differences between secondary defects associated with precipitation in low‐ and high‐carbon specimens suggest that substitutional carbon atoms at 1000 °C act as sites for silicon self‐interstitial condensation near precipitates.