<p>The oxidation mechanisms of high entropy alloys (HEAs) have attracted significant attention in recent years. It is especially important to understand the impact of the microstructures of HEAs on the oxidation processes. We have previously shown that cold working followed by annealing significantly enhances the thermal stability of native oxide solar absorbers on two-phase (FCC + B2) FeMnNiAlCr HEAs, which are promising candidates for cost-effective high-temperature materials in high-efficiency power plants and concentrated solar power systems. In this paper, we further investigate the microstructural evolution after cold working and annealing, as well as their impact on the HEA’s oxidation mechanisms. The thermo-mechanical treatment not only led to a refined microstructure and elemental redistribution producing a more uniform oxide growth, but also accelerated the formation of a new A12 phase from the FCC phase upon thermal cycling. The refined microstructure and the new A12 phase increased the short-cut diffusion pathways, which facilitated the early establishment of a continuous interfacial protective Al₂O₃ layer and ensured its sustained integrity upon long-term thermal cycling. This stable Al₂O₃ layer played an important role in maintaining the adhesion of the oxide scale to the HEA. The Al₂O₃ layer also prevented excessive out-diffusion of transition metal elements, thereby avoiding void formation under the oxide/HEA interface during oxidation. These mechanisms greatly improved the spallation resistance of the FeMnNiAlCr HEA. Overall, this study highlights the significant impact of microstructural engineering on the thermal stability of native oxide on FeMnNiAlCr HEAs.</p>