The kinetics of the high-temperature oxidation of titanium–tantalum carbonitride-based cermets with different Ti/Ta ratios was studied. Isothermal oxidation tests were conducted under static air for 48 h at temperatures between 700 °C and 1200 °C. The oxidation satisfied the parabolic kinetics, characteristic of the existence of a protective oxide layer. The apparent activation energy suggests the rate-controlling process during oxidation is the simultaneous inward and outward diffusion of oxygen and titanium, respectively, through the formed protective layer, consisting mainly of a rutile phase. A higher Ta(V) content in the rutile decreased the oxygen diffusivity due to the reduction of oxygen vacancy concentration.