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High-resistivity detector-grade p-type silicon wafers have been implanted with swift oxygen (O6+) ions under two different conditions. One of the wafers was implanted effectively with a pulsed beam of varied energy (3-140MeV) to a total fluence of 5 × 1015ionscm-2, resulting in a depth-wise near-uniform implantation profile. The other wafer was directly irradiated with a 140MeV steady oxygen beam to the same fluence. Radiation-induced defects produced in the samples and their isochronal annealing behaviours were studied by positron annihilation lifetime spectroscopy and Doppler broadening of positron annihilation radiation measurements. The lifetime spectra of the irradiated samples were fitted with three lifetimes. Trapping model analysis was carried out with the de-convoluted lifetimes to characterize the defect states. The defect-related lifetime τ2 in both the irradiated samples was found to be due to an admixture of divacancy (V 2) and divacancy-oxygen (V2O) complexes. A marked difference in the defect reordering process has been observed between the two samples, which is explained by taking into account the effect of injection annealing by minority carriers which are produced by the energetic beam prior to forming displacement damage. © IOP Publishing Ltd.