A comprehensive understanding of the oxidation of Cu thin films in the low-temperature regime is of fundamental interest and particularly relevant for applications in the fields of micro- and nanoelectronics, sensors, catalysis, and solar cells. The current study reports on the oxidation kinetics of PVD grown Cu thin films (20-150 nm thick) and the oxide phase evolution from Cu₂O to CuO upon thermal oxidation in the temperature range of 100-450 °C. XRD investigations in the laboratory and at the synchrotron show that the oxide phase formation critically depends on the oxidation conditions such as temperature and oxygen partial pressure. The real-time synchrotron XRD measurements reveal that the formation of the CuO phase only starts after complete oxidation of the Cu films to Cu₂O films. In situ resistance measurements were performed to follow the oxide growth rate of Cu₂O on Cu films in the temperature range of 100-300 °C in air and in 10 mbar pO₂. It is found that the oxidation kinetics of Cu films to Cu₂O films follows the linear rate law, which is attributed to surface reaction controlled oxidation. The oxygen dissociation rate at the gas–solid interface is the rate-limiting process. A dramatic decrease in the linear oxidation rate is observed at low oxygen partial pressures. The fundamental differences between the oxidation rate-limiting processes of Cu as compared to other transition metal films are discussed.