It was found that oxygen co-doping at a concentration of 10 atom % in GaMnN films grown by gas-source molecular beam epitaxy under previously determined optimal growth conditions of 3 atom % manganese and a growth temperature of 700 degrees C, did not have an effect on the magnetic properties. Oxygen doping did, however, improve the magnetic properties of films grown under nonoptimal conditions, in accordance with the prediction of Kulatov et al. [Phys. Rev. B, 66, 45203 (2002)]. For all growth conditions examined, oxygen doping greatly decreased the electrical resistivity from highly resistive to similar to 2.5 x 10-(2) ohm-cm. Oxygen co-doping was also found to produce substantial improvement in the thermal stability of the GaMnN, allowing for retention of the room temperature ferromagnetic ordering even after annealing at temperatures up to 600 degrees C. (C) 2004 The Electrochemical Society.