The microstructure, hardness and wear property of Mo-O-N films deposited at different oxygen gas flow rates, fO2, on SUS304 stainless steel were investigated. The hardness, H, of the film increased with increasing fO2 in a low fO2 region (fO2≤0.05sccm) and reached a maximum value of about 32GPa because of grain refinement. With further increasing fO2, the H started to decrease due to the formation of the amorphous phase. Similarly, the effective Young's modulus, E*, of the film increased and then decreased with increasing fO2. It was found that the H/E* of the film could be enhanced by the addition of oxygen. The Mo-O-N film deposited at fO2=0.2sccm showed lower H and E*, but much higher H/E* than Mo-N film because of its considerably lower E*, which was due to the formation of a crystalline/amorphous mixed structure. The film deposited at fO2=0.2sccm with a crystalline/amorphous mixed structure showed the lowest μ in the present study and exhibited less frequent transverse cracking introduced by wear, as compared with that deposited at fO2=0.02sccm with a fine grain crystalline microstructure. These results indicate that the formation of the crystalline/amorphous mixed structure in the Mo-O-N film is effective to enhance the elasticity of the film without the associated large loss of the H, which leads to low μ and good wear resistance on the stainless steel substrate.