Because of its role in the high-temperature oxidation of many alloys, surface deformation has the potential to influence the oxidation response of HEAs. To demonstrate its importance for HEAs, we compared the oxidation response of an equimolar CrMnFeCoNi alloy subjected to different surface finishes. Specimens were either polished to a mirror finish, to minimize surface deformation, or ground with coarse SiC paper, to maximize surface deformation. Prepared specimens were oxidized in air at 800 °C, and the resulting oxide scales and sub-surface microstructures were characterized. The oxidation response differed dramatically as a function of the surface finish. Differences were noted in the oxide scale thicknesses, oxidation products, and internal oxidation. Furthermore, diffusion-induced grain-boundary migration appears to play a key role in the oxidation process of deformed surfaces. The present results point to possible variations in the oxidation response of commercial components due to varying processing conditions and surface finish.