This study investigates the oxidation mechanisms in high-entropy alloys (HEAs) using a model five-element equiatomic CoCrFeNiMn alloy under controlled oxygen conditions. The oxidation-induced surface changes are correlated with each element's interactive tendencies, guided by thermodynamics. Initial oxidation stages follow atomic size and redox potential, with the latter becoming dominant over time, causing composition inversion. The study employs in-situ atom probe tomography, transmission electron microscopy, and X-ray absorption near-edge structure techniques to elucidate the oxidation process and surface oxide structure evolution. The findings deconvolute the mechanism for compositional and structural changes in the oxide film, paving the way for a predictive design of complex alloys with improved resistance to oxidation under extreme conditions. The research highlights the importance of understanding the oxidation behavior of HEAs, which are gaining prominence for structural applications, and provides insights into the complex interactions between elements during oxidation.This study investigates the oxidation mechanisms in high-entropy alloys (HEAs) using a model five-element equiatomic CoCrFeNiMn alloy under controlled oxygen conditions. The oxidation-induced surface changes are correlated with each element's interactive tendencies, guided by thermodynamics. Initial oxidation stages follow atomic size and redox potential, with the latter becoming dominant over time, causing composition inversion. The study employs in-situ atom probe tomography, transmission electron microscopy, and X-ray absorption near-edge structure techniques to elucidate the oxidation process and surface oxide structure evolution. The findings deconvolute the mechanism for compositional and structural changes in the oxide film, paving the way for a predictive design of complex alloys with improved resistance to oxidation under extreme conditions. The research highlights the importance of understanding the oxidation behavior of HEAs, which are gaining prominence for structural applications, and provides insights into the complex interactions between elements during oxidation.