We investigated the oxygen-chemisorbed c(4 × 2) structure on pseudomorphic Fe(111)/Ru(0001) surface using low-energy electron diffraction (LEED) analysis and first-principles density functional theory (DFT) study. The superstructure was prepared by introducing 2 Langmuirs of molecular oxygen gas to the Fe(111)/Ru(0001) surface. We determined the atomic structural model of the c(4 × 2) superstructure using LEED and DFT analysis. The oxygen-chemisorbed c(4 × 2) structure corresponds to an oxygen coverage of 0.25 ML on Fe(111)/Ru(0001), as compared to the coverage of 1 ML for the formation of FeO monolayer on Ru(0001) surface.