Copper thin films of thickness 1000 Angstrom are evaporated on (100) and (111) single crystal Si wafers in the presence of interfacial native silicon oxide (SiOx). The behavior of copper and the mechanism of compound formation at the Cu/Si interface are studied at different temperatures using scanning electron microscopy (SEM), X-ray diffraction (XRD) and Rutherford backscattering (RBS). Annealing in the 600-750 degrees C temperature range leads to the formation of islands of two copper-rich silicides Cu3Si and Cu4Si. On the Si(100), after annealing at 750 degrees C, we observe epitaxially grown Cu3Si crystallites with square and rectangular shapes. However, on Si(111) annealing at the same temperature yields Cu3Si and Cu4Si crystallites with droplet-like shapes and no sign of epitaxy. The presence of oxygen, after heat treatment under vacuum, is closely related to the formation of copper silicide crystallites.