Materials used in the energy sector or aviation must possess good combination of mechanical properties and oxidation resistance. These materials face harsh conditions i.e. high temperature and oxidative atmospheres. This leads to severe degradation of the materials. To overcome this problem, protective coatings are applied. However, this introduces additional costs. Therefore, a new solution is required. Therefore, relatively new kind of alloys, which are high entropy alloys, are of interest of many researchers. In the present work, AlCoCrFeNi high entropy alloys doped with Mo in the amount ranging between 0-5 at.% were investigated to elucidate the effect of Mo addition on AlCoCrFeNi-xMo microstructure, microhardness and high temperature oxidation resistance. It was found that increase in Mo content leads to ordering the microstructure from lamellar, in the case of reference alloy, to mosaic in the case of HEA containing 4 and 5 at.% of Mo. For the latter significant increase in microhardness was found and correlated directly with changes in microstructure. However, oxidation tests revealed that an increase in Mo in addition to HEA worsens the oxidation resistance of the studied materials. This phenomenon was explained by more severe spallation of the alumina scale and subsequent increased formation of internal Al nitrides. Reaction of Al with N-entrapped Al and lowers Al content to a level below critical Al content for transformation from the internal scale to the external scale formation.