The structural stability of an FeO(111) film supported on Pt(111) was studied by density functional theory (DFT) as a function of oxygen pressure. The results showed formation of O-rich phases at elevated O₂ pressures and revealed a site specificity of the oxidation process within the coincidence (Moiré) structure between FeO(111) and Pt(111), ultimately resulting in an ordered pattern of O−Fe−O trilayer islands, as observed by scanning tunneling microscopy (STM). In addition, high resolution STM images revealed a (√3 × √3)R30° superstructure of the FeO₂ islands with respect to pristine FeO(111). This structure is rationalized by DFT in terms of strong relaxations within the Fe sublayer and can be considered as an intermediate state of the FeO(111) transformation into an Fe₂O₃(0001) film.