A systematic study of O implantation into Si at MeV energies and stoichiometric doses is reported. Si substrates were implanted with 1 MeV O ions to doses of 0.73, 1.45, and 2.18×1018cm−2at implant temperatures of 150, 300, and 450 °C. The samples were then subjected to a high temperature anneal (1250 °C for 2 h). Cross‐sectional transmission electron microscopy and Rutherford backscattering combined with channeling were used to study the resultant defect structures. The microstructure after annealing was found to be highly implant temperature dependent while very little dose dependence was observed. In the Si overlayer, twinning and polycrystalline Si were observed at the lowest implant temperature while SiO2precipitation was apparent at all temperatures with precipitate size increasing with implant temperature. In the SiO2layer, Si islands were observed at the high implant temperatures. A comparison is made between MeV and conventional implant energies. At MeV implant energies and stoichiometric doses, crystallinity in the Si overlayer is retained even at implant temperatures as low as 150 °C. Such temperatures are shown to be advantageous for inhibiting SiO2precipitation in the Si overlayer.