The effect of the implantation of oxygen ions on the density of states in the mobility gap of n type hydrogenated amorphous silicon (a-Si:H) is studied using drive-level capacitance profiling, deep-level transient spectroscopy( DLTS), and transient voltage-pulse photocapacitance. After implantation, we observe a broadening of both the conduction and valence band tails and an increase in the dangling bond density near midgap, as well as a shift of the Fermi level toward midgap by approximately 100 meV in two out of the throe films studied. We also observed the addition of a distinct defect band at approximately 0.7 eV below the conduction nd mobility edge in films implanted with oxygen doses greater than 1012 cm−2.