We present a direct evidence that ion implantation through thin (≤5 nm) surface oxide layers is a source of O impurities, which form O-defect complexes during thermal treatment. The impurity-defect complexes are identified by correlating the results from positron annihilation spectroscopy, secondary-ion mass spectroscopy, and Monte Carlo simulations. The O atoms are introduced in the bulk by multiple recoil implantation by the primary ions. The signatures of large VmOn formations are observed at 800 °C, which implies the existence of smaller species at lower temperatures. © 2000 American Institute of Physics.