High precision, single-wavelength optical monitoring of reflectance was shown to be useful in the study of initial oxidation of very thin metal films by low pressure oxygen at room temperature. Thin films of Al, Ni, and Hf metal were sputter-deposited on silicon substrates and their subsequent oxidations were observed at low oxygen partial pressure using a temperature-stabilised laser diode reflectometer. Based on the derived properties of the appropriate metal and oxide films, optical monitoring data were fitted as a multilayer stack comprised of oxide/metal/SiO2/Si. The fitting results show that the exposure to oxygen at a partial pressure of 0.04 Pa forms a certain finite thickness of oxide film on the metal surface. A range of kinetic models such as Deal-Grove, Massoud, and Cabrera-Mott are commonly used to describe the surface oxidation process. However, these models cannot be applied to the initial stage of oxidation, which occurs when a pure metal surface is exposed to oxygen as measured here. Instead, simple chemical reaction kinetics is used to model the time-dependent experimental results of the early stages of oxidation, thus we obtain the equation d(t) = do[1-exp(-t/τ )] and d(t) = do[1-1/(1+t/τ )] for the gas environments of this investigation.