The authors study the structure dependence of epitaxial Pr2O3 layers on Si(001) on the partial pressure of oxygen during growth using reflection high-energy electron diffraction and x-ray reflectivity. The oxygen is necessary to suppress the formation of silicide, but it also inhibits the nucleation of the crystalline oxide phase. High oxygen partial pressure during growth causes the layers to be polycrystalline or even amorphous with hexagonal contributions. It also increases silicate formation at the interface between oxide layer and Si substrate. Thus, control of oxygen during growth is very important to obtain high quality epitaxial Pr2O3 layers on Si.