The effects of the recoil implantation of oxygen on the electrical properties of Si heavily implanted with As through SiO2films have been investigated as a function of annealing temperature, in correlation with the crystal orientation. The concentration profiles of the electrically active As in Si have been compared with those of total As and recoil implanted oxygen measured by secondary‐ion mass spectrometry. It is shown that at temperatures between 650 and 850 °C, both the restriction in the electrical activation and the reduction of mobility are observed in the region where the oxygen concentration exceeds ∼1020O/cm3in (111) ‐oriented Si, while the recoil implantation of oxygen has much less influence on the electrical properties of Si in (100) ‐oriented Si. At 1000 °C the only effect of the recoil implantation is the reduction of mobility in the surface region in (111) ‐oriented Si. The analysis of the defect structures by transmission electron microscopy indicates that the defects in (111) ‐oriented Si after annealing are much denser than those in (100) ‐oriented Si. The isochronal annealing are much denser than those in (100) ‐oriented Si. The isochronal annealing behavior of carrier concentration is correlated with that of defects measured by ESR method.