We investigate by means of first-principles molecular dynamics and total energy density functional calculations the first stages of O deposition oil a SiC surface during thermal oxidation. In particular, we focus on the dynamics of the O-2 molecule dissociation and on its adsorption. The atomic configurations obtained from the dynamics study indicate that intra-bond adsorption plays an important role. We use this input to perform a systematic study of the oxygen adsorption at and near the surface.