We discuss the ab initio modeling of several processes occurring during silicon oxidation with a particular emphasis on the charge state of the diffusing oxygen molecule. For values of the electron chemical potential falling in the range of the silicon band gap, we find that the neutral oxygen molecule is thermodynamically stable far from the Si-SiO 2 interface. When the molecule approaches the interface, the negatively charged state of the oxygen molecule is stabilized and its neutral state destabilized. While these effects both concur to lower the charge transition level in the vicinity of the interface, our estimations suggest that the oxygen molecule remains in its neutral charge state until it reaches the silicon substrate.