Oxygen out-diffusion from CZ-Si or in-diffusion into FZ Si was studied with Secondary Ion Mass Spectrometry. For anneals up to 1200oC , the value of 16O concentration Csurf that develops at the semiconductor surface in bare wafers with ~2 nm thick natural oxide was found to be comparable to that in thermally oxidized wafers with 260 nm thick SiO2. Beyond this temperature, at 1280oC, Csurf in non-oxidised sample appears 5 times lower that in the oxidised one. This means that the spontaneous oxide becomes permeable to the oxygen species and no longer constitutes a barrier preventing the oxygen atoms from direct out-diffusion into the ambient. The effect becomes more pronounced when the specimens were heated to 1150oC in a chamber evacuated to ~ 10-7 Torr. The resultant bare Cz-Si and, to even greater extent, FZ Si with previously in-diffused oxygen exhibited deep oxygen depletion, 10x below the respective values for annealing under atmospheric pressure. Undoubtedly, it is the removal of residual oxides by sublimation that enables oxygen atoms to freely out-diffuse into vacuum.