The forms of the atomic oxygen adsorbed on the Ag(lll) face were investigated using the semi-empirical NDDO/MC method within the modified cluster approximation. Two oxygen positions - surface (over the octahedral hollow) and the subsurface (in the hollow) one - between the first two layers of (111) face were studied. A cross-section of the potential energy for the oxygen penetration into subsurface positions has been computed. A new approach was applied to take into account the lattice relaxation caused by the interaction with adsorbate. The doubly-excited-CI-like correlation corrections to the diffusion and desorption barriers were estimated. The barrier of the oxygen atom diffusion through the surface was shown to be substantially lower than that of the desorption.