Multiple-energy (30-325 keV) O+ implantation into GaN field-effect transistor structures (n similar to 10(18) cm(-3), 3000 Angstrom thick) can produce as-implanted sheet resistances of 4 x 10(12) Omega/square, provided care is taken to ensure compensation of the region up to the projected range of the lowest energy implant. The sheet resistance remains above 10(7) Omega/square to annealing temperatures of similar to 650 degrees C and displays an activation energy of 0.29 eV. No diffusion of the implanted oxygen was observed for anneals up to 800 degrees C.