Using density functional calculations in the generalized gradient approximation, the energetics of competing atomic processes occuring during the oxidation of silicon are evaluated. Simple molecular systems are used to model the breaking of Si-Si and Si-O bonds in the oxide. These calculations suggest that the breaking of Si-Si bonds and the formation of threefold coordinated O atoms are the most favourable transformation pathways of the bonding network of the oxide, in accord with the atomic processes observed during a recent first-principle molecular dynamics simulation.