The role of phase morphology in controlling the high-temperature oxidation of a dual-phase FCC + B2 + BCC AlCoCrFeNi(CuTiZr) high-entropy alloy was investigated using as-cast (AC) and high-temperature annealed (HTA) conditions, identical in composition and phase constitution with near-equal phase fractions and differing only in the spatial arrangement of the two phases. Cyclic oxidation was conducted at 850 °C and 1050 °C, representing diffusion-limited and spallation-dominated regimes, respectively. At 850 °C, the AC condition recorded a parabolic rate constant of 27.0 × 10 ⁻¹³ g ² cm ⁻⁴ s ⁻¹ compared with 102.0 × 10 ⁻¹³ g ² cm ⁻⁴ s ⁻¹ for the HTA condition, and at 1050 °C, subsurface voiding extended to 100–200 μm in the AC condition compared with 500–600 μm in the HTA condition. Neither Al nor Ti reaches the surface in sufficient quantity to form a continuous external Al₂O₃ scale, leaving protection to a Cr₂O₃ framework that codominant spinel renders discontinuous. Oxidation performance is therefore governed by the rate and spatial uniformity with which the B2 + BCC phase supplies Al and Ti to the advancing front. The pathway by which morphology acts switches with temperature: supply path length in the diffusion-limited regime and local composition at the front in the spallation-dominated regime. Spot friction processing refined both phases to approximately 1.9 μm without phase transformation or change in phase balance and reduced mass loss at 1050 °C by more than 50% relative to the AC and HTA conditions within a 20-h window. These results establish phase morphology as an independent and controllable design parameter for high-temperature oxidation resistance in multiphase high-entropy alloys.