Damages induced by 69 MeV oxygen ions (fluence: 1.3×1014ions cm−2) in gold–silicon‐nitride–silicon metal‐insulator‐semiconductor (MIS) structures were studied by measuring the pre‐ and postirradiation junction and interface characteristics. Amount of the space‐charge produced at the silicon–silicon‐nitride interface was estimated by measuring capacitance‐voltage characteristics and the energies of the defects were measured with the technique of deep‐level transient spectroscopy. Two new defect levels having energies (Ec−0.34) eV and (Ec−0.63) eV were found in addition to the defects detected in the unirradiated devices. The values of the surface recombination velocities, measured with a variable‐energy electron beam, indicate that the radiation‐induced defect concentration on the surface was negligible; however, about 7% degradation in the lifetime of minority carriers in the bulk was observed. The results indicate that oxygen ions can be used to modify the junction characteristics of MIS structures while keeping the surface characteristics of the structure almost unaltered.