By using the density-functional theory, we have systematically investigated the adsorption of oxygen on Pt(1 1 1) surface for a wide range of coverages (from 0.11 ML to 1.00 ML) and adsorption sites. In the view of adsorption energy maximization, we found that the fcc site is the most stable site for oxygen adsorption, which is always slightly energetically favorable than the hcp site, and for these two sites, the adsorption energies decrease as the increasing oxygen coverage due to the increasing repulsive lateral interactions in the overlayer O adatoms. Except for coverage of 1.00 ML, the oxygen-induced lateral relaxations and bucklings are found in the outermost substrate Pt layers. Taking into account the effects of the chemical environment as a function of the relative richness in oxygen, we obtained that oxygen can form two stable configurations in the fcc site adsorption: the 0.25 ML configuration in oxygen-poor conditions, and the 0.50 ML case in oxygen-rich environments. The work function and the surface dipole moment are also studied and analyzed. Electron transfer between the first layer Pt and the O adatoms indicates the O-Pt chemical bond shows some degree of ionic character. In addition, the hybridization between O 2p and Pt 5d orbitals, especially at oxygen coverage of 1.00 ML, implies the O-Pt bond also shows some degree of covalence character. Moreover, with the increasing oxygen coverage, more Pt 5d states are empty thus weakening the binding of O/Pt(1 1 1) systems at higher coverage, and the density of states at the Fermi level for the O/Pt(1 1 1) system decreases thereby the metallic character is weakened. (C) 2010 Elsevier B.V. All rights reserved.