By taking the service conditions of a high salinity reservoir water environment with high temperature, high pressure, high-Ca²⁺ concentration and O₂-CO₂ coexistence in a western oilfield in China into account, the evolution of corrosion products film and corrosion behavior of C110 steel in different oxygen partial pressures (0-0.4 MPa) were investigated using a high-temperature/high-pressure reactor and an electrochemical testing system. The results showed that as the oxygen partial pressure increased, the uniform corrosion rate of the steel significantly rose from 0.59 to 3.56 mm/a, with intensified localized corrosion. In pure CO₂ environment, a highly resistive composite film (inner layer FeCO₃ and outer layer CaCO₃) with protective properties was formed, resulting in low corrosion current density. In conditions of coexisting O₂-CO₂, additional FeO(OH) and Fe₂O₃ appeared in the corrosion products. Notably, under high oxygen partial pressure (P{}_{{\mathrm{O}}_{\mathrm{2}}} = 0.4 MPa), the corrosion product film was composed of a top layer of porous Fe₂O₃, intermediate CaCO₃ layer with dispersed Fe₂O₃ particles, and discontinuous inner layer FeCO₃. This structural evolution drastically reduced the film resistance and increased the corrosion current density. Moreover, Fe₂O₃ promoted the precipitation of CaCO₃. Under the synergistic effect of O₂ and CaCO₃, the protective FeCO₃ film diminished, and CaCO₃ exhibited both inward and outward growth patterns. Additionally, the coupling of Ca²⁺ and Cl^- exacerbated localized corrosion. This study will provide in-depth theoretical insights into the corrosion mechanisms of C110 steel in coditions of high-salinity, high-Ca²⁺ concentration, and O₂-CO₂ coexistence etc., offering important reference for corrosion protection design in harsh production well environments.