We model the fundamental kinetic processes occurring during silicon oxidation at the atomic scale. We first focus on the diffusion of the, neutral O-2 molecule through the oxide layer. By combining ab initio and classical simulations, we derive a statistical description for the O-2 potential energy landscape in the oxide. Statistical distributions are then mapped onto lattice models to investigate the O-2 diffusive process in the bulk oxide and across an oxide layer at the Si(l 00)-SiO2 interface. We find that the diffusion of O-2 is a percolative process, critically influenced by both energetical and geometrical features of the potential energy landscape. At the interface, the occurrence of a thin densified oxide layer in contact with the substrate limits percolative phenomena and causes the O-2 diffusion rate to drop below its value for ordinary amorphous SiO2. Then, we use first-principles calculations to address the kinetic processes occurring in the proximity of the Si(100)-SiO2 interface. We first focus on the energetics of negatively charged oxygen species in the oxide, and on the diffusive and dissociative properties of the charged molecular species. We find that negatively charged oxygen species incorporate in the oxide at Si sites, giving rise to additional Si-O bonds and important network distortions. Finally, we focus on the oxidation reaction at the Si(100)-SiO2 interface. We find that the O-2 oxidation reaction occurs by crossing small energy barriers, regardless of the spin or charge state of the molecular species. Our findings are consistent with kinetics pictures of the silicon oxidation process entirely based on diffusive phenomena.