<p>This work reveals the oxygen partial pressure–regulated oxidation behavior of a Ni–26W–6Cr superalloy at 700–850 °C and elucidates the mechanism underlying its insufficient oxidation resistance. At 700–775 °C, oxidation produces a duplex scale comprising an outer NiO layer and an inner mixed oxide layer, whose evolution is strongly influenced by Cr content and W-induced suppression of Cr diffusion. At 850 °C, early-stage Cr₂O₃ volatilization disrupts scale integrity and locally elevates the oxygen partial pressure, thereby promoting the formation of an intermediate Ni(Mn, Cr)₂O₄ spinel layer. This spinel layer impedes oxygen transport and drives the compositional transformation of the inner scale from NiWO₄ + NiCr₂O₄ to NiWO₄ + Cr₂O₃. This study establishes a mechanistic link between oxygen partial pressure fluctuations and oxide scale evolution in multi-component alloys, offering insights for designing alloys with improved high-temperature oxidation resistance.</p>