We present measurements of the oxidation level (Cu+ and Cu2+ concentration) of copper thin films deposited using a new type of oxygen plasma source. This oxygen plasma source is operated under ultrahigh vacuum, and allows one to oxidize copper up to nearly pure Cu2+ oxide at rather low temperature (<500 °C) and low molecular oxygen background pressure (2×10−6 Torr). High growth rate and high substrate temperature promote the formation of Cu+ oxide, whereas Cu2+ oxide is mainly obtained at low growth rate (0.5 Å s−1) and low substrate temperature (450 °C). The active species for oxidation is mostly atomic oxygen produced in the plasma. When the plasma is off, while the oxygen flux remains unchanged, no copper oxide is detected in the films.