We investigate the effects of oxygen on the thermal diffusion of germanium atoms, implanted inside a thermally grown SiO2 layer, during high temperature processes (1100°C, 60 minutes). The impact of oxygen presence on Ge diffusion is studied as a function of its origin, as it can come either from the annealing atmosphere (extrinsic source) or from the SiO2 matrix itself (intrinsic source). 18O labeling of the oxygen either in the annealing atmosphere or in the silica substrate, together with isotopically sensitive Ion Beam Analysis (IBA), shows a clear oxygen-dependence in germanium diffusion. This is especially so when oxygen is present in the annealing atmosphere, where it is responsible for an enhancement of germanium out-diffusion and redistribution into several peaks during annealing, through the formation of GeO molecules. A new three-process model is proposed to explain the impact of a contaminated atmosphere on Ge redistribution. This is notably shown that a third Ge peak arises at the sample surface when the annealing atmosphere is contaminated by oxygen. This peak formation is explained by the oxidation of Ge present at the vicinity of the surface by oxygen coming from the annealing atmosphere. This is also shown that O2 molecules can diffuse in depth, with a coefficient of diffusion D~10^-9 cm^2/s, until the densities of Ge and irradiation-induced defects increase, causing the progressive oxidation of Ge in depth and the restoration of the SiO2 stoichiometry.