We have investigated structures of FenPtm (FePt) bimetallic systems, starting with small clusters with N= n + m <= 6, based on density functional theory (DFT), using the PBE functional. The stability of all clusters increases with the increase of Fe-Pt bonds. It was found that the O-2 molecule adsorbs on the bridge site for the bare Pt-N clusters and on certain Fe top sites for FePt clusters. The O-binding strength is reduced for clusters which have <50% Fe composition. For comparison with experimental results reported, we also calculated four larger FePt nanoparticle systems, which showed similar trends. (c) 2015 Elsevier B.V. All rights reserved.