A gaseous mixture of CO2 and C5F10O has been widely investigated as one of the alternative gases for SF6. Due to the better electron attachment ability of oxygen molecule O2, it is generally expected that adding of O2 gas would improve the dielectric strength of arc quenching and insulating gas for electric power equipment. Therefore, in this paper, the dielectric breakdown properties of hot CO2/O2/C5F10O gas mixture were numerically estimated. The calculation results revealed that the critical electric field strength of the CO2/C5F10O gas mixture is disappointingly decreased by the addition of O2 gas, especially in the temperature range of 3000 K to 1000 K. This is brought about by the increase of ionization reaction of CO2 molecules with an increase in the O2 concentration in the arc‐quenching gas of CO2/O2/C5F10O. © 2024 Institute of Electrical Engineers of Japan and Wiley Periodicals LLC.