Abstract: Co–Ni-based superalloys are known for their capability to function at elevated temperatures and superior hot corrosion and thermal fatigue resistance. Therefore, these alloys show potential as crucial high-temperature structural materials for aeroengine and gas turbine hot-end components. Our previous work elucidated the influence of Ti and Ta on the high-temperature mechanical properties of alloys. However, the intricate interaction among elements considerably affects the oxidation resistance of alloys. In this paper, Co–35Ni–10Al–2W–5Cr–2Mo–1Nb–xTi–(5−x)Ta alloys (x = 1, 2, 3, 4) with varying Ti and Ta contents were designed and compounded, and their oxidation resistance was investigated at the temperature range from 800 to 1000°C. After oxidation at three test conditions, namely, 800°C for 200 h, 900°C for 200 h, and 1000°C for 50 h, the main structure of the oxide layer of the alloy consisted of spinel, Cr₂O₃, and Al₂O₃ from outside to inside. Oxides consisting of Ta, W, and Mo formed below the Cr₂O₃ layer. The interaction of Ti and Ta imparted the highest oxidation resistance to 3Ti2Ta alloy. Conversely, an excessive amount of Ti or Ta resulted in an adverse effect on the oxidation resistance of the alloys. This study reports the volatilization of W and Mo oxides during the oxidation process of Co–Ni-based cast superalloys with a high Al content for the first time and explains the formation mechanism of holes in the oxide layer. The results provide a basis for gaining insights into the effects of the interaction of alloying elements on the oxidation resistance of the alloys they form.