The development of atomic imperfections within oxide scales from high-temperature oxidation of heat-resistant alloys significantly limits the self-protectiveness of the surface oxide, contributing to the failure of energy generating system components such as turbines, engines, and heat exchanges. Directly probing the dynamics of such atomic defects is challenging because of the extreme thermochemical conditions of high-temperature oxidation. CO2 is ubiquitously present as a naturally occurring product of the combustion of petrochemical fuels and causes significant degradation of the critical alloy components of energy and power systems. CO2 has been found to be highly corrosive, causing significant oxidation of steels of the critical components of the power system. The chromia-forming steels that are used widely and successfully in air undergo a breakaway of the protective Cr2O3 oxide layer in the CO2 atmosphere. Using environmental transmission electron microscopy observations, we directly capture atomic-scale dynamics of vacancies in growing Cr2O3 film during high-temperature oxidation of NiCr alloy in CO2. Coordinated with theory modeling, we delineate the atomistic mechanisms associated with the effect of interstitial carbon derived from CO2 on promoting the formation, migration and clustering of atomic vacancies to result in the enhanced alloy oxidation. The identified oxidation mechanism may find broader applicability in utilizing the atmosphere to tune the formation and evolution of atomic-scale defects, thereby affecting the mass transport properties of the growing oxide scale.