The rate of oxide growth on Si, in dry atmosphere, shows two regimes: in the first (for thin oxide up to approximately 30 nm) the oxide is formed very quickly while in the second the rate decreases markedly as the thickness of the oxide layer increases. To describe the overall oxide growth we make use of Fick's second law but, at the same time, we allow for inhomogeneities and preferred diffusional pathways within the SiO2 as suggested, for example, by Revesz et al. in 1986 or, equivalently, for a possible ''granular network'' as envisaged by Duval et al. in 1990 and by Verdi and Miotello in 1992. To do this we introduce a depth-dependent diffusivity as is appropriate to a fractal geometry. This framework intrinsically explains the two experimentally observed regimes for oxide growth without empirically introducing a depth-dependent oxygen diffusivity as done in current literature. Finally we show how some reported features of radiation effects during oxide growth may help in a deeper understanding of the oxidation process itself.