Sulfur hexafluoride (SF₆) is widely used in the power industry because of its excellent insulation and arc extinguishing performance; however, as the global environment is deteriorating, the need to replace SF₆ is becoming significantly critical. In recent years, C₅F₁₀O has received extensive attention as a potential alternative to SF₆. In this study, a part of N₂ in C₅F₁₀O/N₂ was replaced by O₂, and the breakdown voltages of C₅F₁₀O/N₂/O₂ at different oxygen concentrations under a slightly uneven electric field were tested. The dispersion of breakdown voltage and the discharge decomposition components of C₅F₁₀O/N₂/O₂ with different oxygen concentrations were analysed. It was found that as the oxygen concentration increased, the breakdown voltage of C₅F₁₀O/N₂/O₂ with 15 kPa C₅F₁₀O at 0.2 MPa increased, and the dispersion of the breakdown voltage became worse. When 0.5% O₂ or more O₂ was added to the C₅F₁₀O/N₂ gas mixture, the carbon precipitates on the electrode surface disappeared. As the oxygen concentration continued to increase, another characteristic component, CF₂O, could be detected, whereas C₂F₄ and C₃F₆ disappeared. It is believed that O₂ can inhibit the formation of C₂F₆, C₃F₈, C₄F₁₀, and C₃F₇H. Therefore, it is recommended to use oxygen as the second buffer gas for the engineering applications of C₅F₁₀O. Moreover, the ratio of C₅F₁₀O to O₂ is recommended to be 1 : 1.