In recent years, high-entropy alloys (HEAs) received great research interest due to their excellent performance. This study investigates the oxidation behavior of transition metal (TM) HEAs subjected to both static oxidation and cyclic oxidation under 900 and 1000 °C in the ambient atmosphere. Four HEAs are manufactured for the study. The oxidation kinetics, microstructures of oxidation products, and phases evolution under high-temperature oxidation conditions are studied in order to better understand the degradation of HEAs during oxidation. The results indicate the addition of Al has promoted alloy oxidation resistance by forming a protective oxide layer, and Cr element also contributes to this protective scale formation. The Ni2.1 eutectic alloy is shown to be the most oxidation resistant, followed by H4Cu10Al10, H4Cu15Al5, and H4Cu20. The excellent high-temperature performance of Ni2.1 in both static and cyclic conditions can be attributed to both protective scale formation and high stability due to the eutectic structure. Under cyclic conditions, alloys encounter severe oxidation owing to the enhancement of oxide growth and peeling, which leads to continuous oxidation of alloy at higher temperatures.