We perform first-principles calculations to investigate the electronic structure of native defects in various oxide dielectrics to address the impact of defects on GaN/oxide metal–oxide–semiconductor (MOS) devices. We calculate defect formation energies in Al2O3, HfO2, and LaAlO3, as a function of the chemical potentials and of the Fermi-level position in the band gap. By aligning the conduction-band and valence-band edges of these oxides to those of GaN, the role of native defect as charge-trap or fixed-charge centers in GaN-based MOS devices is examined. We find that oxygen vacancies in the oxide dielectrics result in charge-state transition levels near the GaN conduction-band edge. They can therefore introduce border traps and/or leakage current through the gate dielectric in the n-GaN/oxide MOS devices such as MOS field-effect-transistors. While the transition levels for other defects are well away from the GaN conduction-band edge, these defects are stable in non-neutral charge states and thus act as sources of fixed charge in MOS devices.