The isothermal phase transformation behavior of thermally grown oxide scale of FeO on high-purity Fe was investigated under conditions of low oxygen potential. The phase transformation comprised three reactions, namely Fe3O4 precipitation, magnetite seam formation, and the eutectoid decomposition. The nucleation rate of Fe3O4 precipitation decreased with decreasing oxygen potential, whereas the incubation period for the eutectoid decomposition became longer. In contrast, magnetite seam formation was independent of the oxygen potential. The overall phase transformation was found to reach completion faster in an atmosphere with a lower oxygen potential. The effect of the oxygen potential on the phase transformation of FeO scale was rationalized by the different degree of supersaturation for Fe3O4 precipitation, which plays a crucial role in the eutectoid reaction.