The significant difference in the formation of thermally grown oxide and the buried oxide (BOX) produced by high dose oxygen implantation in silicon (SIMOX) is described. The different sources of the defects in the Si overlayer and the SiO2 buried layer produced during implantation and annealing treatment are discussed. The trapping of Si in the BOX results in the formation of Si islands and strained Si-Si bonds. Stacking fault (SF) complexes are formed during the high temperature annealing at the interface between the Si overlayer and the buried oxide due to the dissolution of the SiO2 precipitates. The morphology and density of the SF complexes under different implantation conditions are studied. The role of the sacrificial oxidation of the Si overlayer in the defect generation before and after high temperature annealing is discussed.