Abstract: C₄F₇N/CO₂/O₂ gas mixture has become the most potential alternative to SF₆ gas insulation medium in the power industry because of its excellent environmental protection and insulation properties. Although the addition of O₂ can enhance the insulating properties and chemical stability of C₄F₇N/CO₂ gas mixtures to a certain extent, the pattern of the influence of O₂ addition and its change in volume fraction on the generation characteristics of gases and solid by-products under the action of PD of the gas mixtures is not clear. In this paper, experiments on C₄F₇N/CO₂/O₂ 96 h PD and its decomposition characteristics were carried out through metal protrusion defects in a pin-plate electrode simulation device. It is found that the addition of 2%~4% O₂ to the C₄F₇N/CO₂ gas mixture can significantly suppress the generation of most of the by-products, and the cumulative discharge per second and the average discharge per second will increase dramatically at O₂ volume fractions greater than 6%, leading to increased decomposition of the gas mixture. C₄F₇N/CO₂/O₂ shows obvious circular solid precipitates in the center area of the plate electrode surface under long-term PD action, and the main components include metal oxides (CuO), fluoride (CuF, CF_x), carbon oxides (CO, CO₂), and nitrogen oxides (NO, NO₂). The addition of 4% O₂ has a negligible effect on the elemental composition and valence state of PD solid precipitates, but can reduce its production to a certain extent. The corrosion effect of O₂ on metal conductors in mixed gas is weaker than that of C₄F₇N.