Atomistic simulations employing dynamic charge transfer between atoms are used to investigate ultra-thin oxide growth on Al(100) metal substrates in the presence of an ac electric field. In the range of 1–10 GHz frequencies, the enhancement in oxidation kinetics by ∼12% over natural oxidation can be explained by the Cabrera–Mott mechanism. At field frequencies approaching 0.1–1 THz, however, we observe a dramatic lowering of the kinetics of oxygen incorporation by ∼35% compared to the maximum oxidation achieved, which results in oxygen non-stoichiometry near the oxide–gas interface (O/Al ≈ 1.0). This is attributed to oxygen desorption from the oxide surface. These results suggest a general strategy to tune oxygen concentration at oxide surfaces using ac electric fields that could be of interest in diverse fields related to surface chemistry and applications such as tunnel barriers, thin dielectrics and oxide interfaces.