An understanding of the reaction mechanisms of oxide/metal bicatalysts is important for their design to achieve better catalytic performance. Using the density functional theory calculations based on the GGA+U approach, the ferrous oxide (FeO) clusters on Pt(111) were systematically investigated as a model of oxide/metal bicatalyst since they showed high catalytic capacity on the preferential oxidation of carbon monoxide. Our calculations showed that the role of the coordinatively unsaturated ferrous (CUP) atoms at the edges of the FeO/Pt(111) interface was to help the dissociative adsorption of oxygen molecules. The oxygen atoms at the edges in the intermediate were more chemically active according to the analysis of their electronic properties. They can selectively attract the carbon monoxide molecules to oxide them. After the desorption of carbon dioxide molecules, the CUP atoms at the edges can be reproduced. The high efficiency and selectivity of FeO/Pt(111) bicatalysts were, therefore, explained using our theoretical results.