<p>The oxidation resistance of Ni-Co-based superalloys is a critical factor affecting service life under conditions. As an important alloying constituent, Cr plays a governing role in modulating the oxidation behavior by promoting the formation of protective oxide scales. To reveal the relationship between Cr content and oxidation behavior, this study systematically investigates the isothermal oxidation behavior of Ni-20Co-based superalloys with Cr contents of 5, 15, and 25% (wt.%) at 900 °C. The alloys with 5% Cr and 15% Cr contents exhibit identical oxide phase compositions. A mixed CoO-NiO layer is formed in the outer oxidation zone with composite Cr₂O₃-Al₂O₃ oxides in the inner oxidation region. Moreover, the 15% Cr alloy develops a continuous Cr₂O₃ layer within the inner oxidation zone, in contrast to the 5% Cr alloy. This continuous barrier effectively impedes oxygen diffusion into the substrate, thereby enhancing the oxidation resistance of the alloy. When the Cr content is further increased to 25%, the oxidation behavior of the alloy demonstrates significant modifications. The outer oxide layer is transformed from a CoO-NiO mixed phase to a dense and continuous Cr₂O₃ protective layer, imparting high-temperature oxidation resistance to the Ni-Co-based superalloys.</p>