The longer term oxidation behaviour of a low carbon, low silicon steel in 17H2O–N2 at 900°C was examined. The oxidation kinetics exhibited a multi-stage pattern with the first stage being initially linear and then transitioned to parabolic with a rate constant two orders of magnitude smaller than prediction. This was followed by an accelerated stage, and then a second parabolic stage with a rate constant much closer to that of iron and steel oxidation in air or oxygen. A single-phase wustite layer was observed throughout the oxidation period. Using Wagner’s scaling equation, the effective oxygen potential (oxygen potential achieved at the outer surface of the wustite layer) was calculated. During the first parabolic stage, this potential was extremely low, leading to a very low oxidation rate. It was then increased continuously through the accelerated oxidation stage to a much higher level at the second parabolic stage. The possible mechanism responsible for the observed trend of effective oxygen potential change and the corresponding trend of oxidation kinetics was discussed.