p-type Cu-doped NiO (NiO:Cu) thin films, a transition metal oxide as hole transport layer material, were fabricated using sputtering technology at a high substrate temperature. It is found that cooling oxygen pressures (Pc-o2), with the range of 10(-4)-2 x 10(4) Pa, modulating oxygen vacancies in the films, have an important impact on the property of NiO:Cu films through Ni and Cu oxidization level. The NiO:Cu film with a proper cooling oxygen pressure, 10(3) Pa in this work, reveals high hole concentration and transmittance, leading to a relatively high photovoltaic performance in the NiO:Cu/c-Si heterojunction solar cell. The lack of oxygen, e.g., cooled at oxygen pressure of 10(-4) Pa, reduces Ni3+/Ni2+ and Cu1+/Cu2+ ratio in films, leading to the low hole concentration. The hole concentration is found to be related to open circuit voltage (V (oc)) and fill factor (FF) in NiO:Cu heterojunction solar cell devices. But the excess of oxygen, e.g., cooled at oxygen pressure of 10(4) Pa, slightly increases Ni2+ and Cu2+ content, which is not helpful to the good device performance. The optical band gap of the NiO:Cu film increases with increase in Pc-o2. The external quantum efficiencies of the devices are found to be fully consistent with the variation trend of optical properties of NiO:Cu films, which explained the change in short-circuit current (J (sc)) of the NiO:Cu/Si heterojunction solar cells. Therefore, this work enlightens a detailed understanding the effect of oxygen on NiO:Cu film photoelectrical properties and provides novel strategies for optimizing NiO:Cu/c-Si heterojunction solar cell performances.