The oxidation rate of resin-bonded magnesia-carbon refractory, MgO-C, containing 5 mass % carbon, was measured continuously with a thermobalance in the temperature range from 1273 to 1823 K in N-2-O-2 and Ar-O-2 mixed gases with 2.1x10(4) and 3.7x10(2) Pa of oxygen partial pressure, pO(2), respectively. The effect of the oxygen concentration in the atmospheric gas on the oxidation rate for the MgO-C refractory has been investigated. The value of the effective diffusion coefficient D-e of O-2, which diffuses through the porous decarburized layer, decreased abruptly when the oxidation temperature went up to 1823 K under 2.1x10(4) Pa of pO(2) in N-2-O-2. When the value of pO(2) in the atmospheric gas was 3.7x10(2) Pa, the abrupt decrease of D-e occurred at a lower oxidation temperature of 1673 K. At low pO(2), a dense layer, probably MgO, formed near the surface of the refractory. It was speculated that the recession speed of the graphite phase was slower due to the formation of this dense phase.