The GaMnN and GaMnN: O films were obtained by annealing at different NH3 flow rates. When the NH3 flow rate was decreased to 20 sccm, the oxygen was co-doped into GaMnN film and substituted the few host atoms, which were confirmed by the X-ray Photoelectron Spectra. Moreover, the oxygen co-doping in GaMnN film drastically enhanced the gain of the ferromagnetic (FM) state but oxygen as shallow donor in GaN offers little carriers. Hence, the enhanced FM state is not due to increase of carrier concentration. On optical properties, the oxygen co-doping in GaMnN film will suppress the blue luminescence centered near 2.8 eV and make a blue shift of the photoluminescence spectra. Copyright © 2013 John Wiley & Sons, Ltd.