Numerically-evaluated space-charge effects on the ionic current determining oxide growth rate show modification factors which can change markedly with oxide thickness. The electric-field modification factors can typically change the rate of oxidation by an order-of-magnitude. In addition to affecting rate, the thickness-dependence of the modification factors has an important bearing on the growth law. The results of new numerical calculations are compared to, and found to be in reasonable accord with, published analytical developments for the thin-film and the thick-film limits. The voltages across the growing oxide are likewise evaluated, and compared to voltages predicted analytically for these limits.