(TixTa1−x)(C0.5N0.5)–Co-based cermets with various Ta contents (x = 0, 0.01, 0.05, 0.1 and 0.2) were oxidized at 900 °C for 48 h in static air. A parabolic rate law, which is indicative of the formation of a protective oxide layer, was observed for all samples. The multi-layered oxide scale and the internal oxidation region that formed as the oxidation progressed toward the interior of the cermet specimens were characterized using XRD, SEM and EDS. The enhanced oxidation resistance achieved in cermets composed by a hard component with stoichiometry Ti0.95Ta0.05C0.5N0.5 may satisfy the optimal requirements for many applications in the field of cutting tools.