Heptafluoro-iso-butyronitrile [(CF3)(2)CFCN, in abbr., C4] with CO2 gas mixture is a promising dielectric candidate to replace sulfur hexafluoride (SF6) for the sake of environmental concern. Detailed mechanisms for the reactions of C4 with atomic oxygen in both triplet and singlet due to dissociation of CO2 were proposed using high-level ab initio methods including density functional theory, quadratic complete basis set, multireference Rayleigh-Schrodinger perturbation theory, and state-averaged multiconfiguration self-consistent field. The reaction of C4 with O(P-3) proceeds via the stepwise C-O and N-O addition/elimination and direct displacement mechanisms. The reaction paths are bifurated into cis and trans conformations. The predominant product channel is the C-O association with the barriers 6.1-6.8 kcal/mol to form the C3F7C(O)N intermediates and followed by the C-C bond cleavage to produce C3F7 and NCO radicals. On the singlet surface, the three-center intramolecular conversion to form the C3F7OCN isomer becomes competitive. The singlet-triplet intersystem crossing is favorable due to the significant spin-orbital coupling. Master equation calculations were carried out to obtain the temperature- and pressuredependent rate coefficients and yields of the O(P-3) + C4 reaction and were compared with the analogous reactions of nitriles. It was found that only the NCO + C3F7 product channel is significant. The overall rate coefficients are pressure-independent in the range 1-10 atm and can be expressed by k(infinity)(T/K)=(6.41 +/- 0.22) x 10(-13)(T/298)(1.30 +/- 0.01)e(-(3241.5 +/- 24.5)/T) cm(3) molecule(-1 )s(-1) . The present theoretical work provides useful insights into the breakdown diagnostic of the C4/CO2 insulation gas.