Recent advancements in high-entropy alloys (HEAs) and high-entropy materials (HEMs) show promising potential for different fields of applications. The emergence of HEAs and HEMs has gained significant interest for their exciting nature and properties. As they consist of five or more elements in considerable amounts, properties vary depending on the synergistic effect of combinations of elements. By selecting proper elements and manufacturing methods, better properties can be tuned. Although many unique behaviors of HEAs and HEMs are reported due to their mixing entropy, sluggish diffusion, severe lattice distortion, and multi-metallic cocktail effects, it is necessary to summarize the data to map their feasibility and potential. For example, the combined properties of high thermal stability, thermal fatigue, creep resistance, higher stiffness, and better corrosion resistance for elevated-temperature environments in aerospace applications are pursued. Moreover, gaining the environmental compatibility and longevity of service-life-oxidation behavior of these materials is one of the crucial aspects and, hence, has been recently explored. Therefore, this Research Update aims at summarizing the recent developments and findings in oxidation behavior and highlighting the challenges and controversies for future research perspectives, particularly, on the sustainability for different applications. Moreover, besides the bulk structure, the performance of the HEAs/HEMs coatings is also reviewed.