A wide information gap exists between our present atomic-scale knowledge of metal oxidation derived from conventional ultrahigh vacuum (UHV) surface science experiments and the oxidation mechanisms obtained from the growth of bulk oxide thin films under technologically relevant realistic (or near-) atmospheric conditions. To bridge this pressure gap, we present an in-situ transmission electron microscopy (TEM) study of the initial oxidation stage of Cu(100) and Cu-Au(100) surfaces where the oxygen partial pressure varies from 5x10-4 to 150 Torr. For Cu(100), with increasing oxygen partial pressure (pO2), the nucleation density of the oxide islands increases and so does the growth rate of the oxide islands. As the pO2 continues to increase, a transition from epitaxial cube-on-cube Cu2O islands to randomly oriented oxide islands is observed. A kinetic model based on the classic heterogeneous nucleation theory is developed to explain the effect of oxygen partial pressure on the oxide orientation. It is shown that such a transition in the oxide nucleation orientation is related to the effect of oxygen pressure on the nucleation barrier and atom collision rate. The Cu-Au(111) alloy revealed the same oxygen pressure dependency of the oxide nucleation orientation as pure Cu oxidation.