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A combination of oxygen redox and Mn-based oxides would be the best option for high-energy density Li-ion batteries crucial for a sustainable society. The disordered rock-salt Li4Mn2O5 was recently reported to display very large capacity of 460 mAh/g with relative reversibility. Previous studies proposed the involvement of lattice oxygen redox in such intriguing electrochemical performance, whereas no direct evidence was presented. To clarify the charge compensation mechanism, we systematically investigated the evolution of the electronic structure of both Mn and O upon cycling via Mn/O K-edge XAS spectroscopy. Mn K-edge XAS unequivocally demonstrates the participation of Mn redox upon the initial stages of charging, yet changes are arrested at the high potentials, while O continues to evolve according to O K-edge XAS. Upon discharging, both Mn and O partially recover to their pristine states. The results highlight the significance of a disordered structure in maintaining the reversible redox chemistry of both transition metals and oxygen to design cathode materials with high energy density.