The results are presented of a fundamental study of electrically active damage introduced in silicon diodes by irradiation with the fission products resulting from the decay of a 252Cf source and with high energy protons. The influence of the oxygen content of the silicon substrate and the irradiation type on the damage formation is investigated using deep level transient spectroscopy. A radiation hardening effect by interstitial oxygen is observed. Bom types of irradiation create the same dominant defect levels but with different relative densities. The identification of the induced deep levels are confirmed by isochronal annealing results.