The adsorption properties and the bonding nature of atomic oxygen adsorbed on the three-hollow sites of Pt-9 and Pt-25 cluster models of the Pt(111) surface have been studied by means of ab initio wave functions. Correlation effects have been introduced by various methods : single and multireference second-order perturbation theory, multireference configuration interaction, and density functional corrections to the Hartree-Fock energy using a variety of expressions of density functionals. While the adsorption and vibrational properties of adsorbed oxygen are sufficiently well described at the Hartree-Fock level, the adsorption energy is completely wrong its the system is unbound in the one-electron approximation while it is strongly bound at the correlated level, The use of correlation functional based on the Hartree-Fock density is not sufficient to obtain a correct estimate of the strength of the bond. We will also show that the bonding of oxygen to Pt has a partial ionic character and the effect of correlation is to enhance the importance of covalent contributions to the bonding.