The reversibility and cyclability of anionic redox in battery electrodes hold the key to its practical employments. Here, through mapping of resonant inelastic X-ray scattering (mRIXS), we have independently quantified the evolving redox states of both cations and anions in Na 2/3 Mg 1/3 Mn 2/3 O 2 . The bulk Mn redox emerges from initial discharge and is quantified by inverse partial fluorescence yield (iPFY) from Mn-L mRIXS. Bulk and surface Mn activities likely lead to the voltage fade. O-K super-partial fluorescence yield (sPFY) analysis of mRIXS shows 79% lattice oxygen redox reversibility during the initial cycle, with 87% capacity sustained after 100 cycles. In Li 1.17 Ni 0.21 Co 0.08 Mn 0.54 O 2 , lattice oxygen redox is 76% initial-cycle reversible but with only 44% capacity retention after 500 cycles. These results unambiguously show the high reversibility of lattice oxygen redox in both Li-ion and Na-ion systems. The contrast between Na 2/3 Mg 1/3 Mn 2/3 O 2 and Li 1.17 Ni 0.21 Co 0.08 Mn 0.54 O 2 systems suggests the importance of distinguishing lattice oxygen redox from other oxygen activities for clarifying its intrinsic properties. Battery cathodes based on 3d-transition-metal oxides need viable improvements in their energy density. Recent proposals of O redox have enabled conceptual possibilities, although the assessment of its reversibility remains elusive. This work reports independent and direct quantifications of the evolving O-2p and Mn-3d redox states through O-K and Mn-L mapping of resonant inelastic X-ray scattering (mRIXS). The high reversibility of the lattice O redox in Na 2/3 Mg 1/3 Mn 2/3 O 2 (79%) and Li 1.17 Ni 0.21 Co 0.08 Mn 0.54 O 2 (76%) is revealed during the initial cycle. While Na 2/3 Mg 1/3 Mn 2/3 O 2 displays decent O-redox capacity retention (87% after 100 cycles), the Li-rich system shows significant decay (44% after 500 cycles). We demonstrate direct quantifications of the reversibility of lattice O redox through photon-in-photon-out bulk-sensitive mRIXS. The quantification results directly show that the reversibility of lattice O redox could be very high in both Li-ion and Na-ion batteries. Reversibility of lattice oxygen redox holds the key to its practical employments, especially in 3d transition-metal compounds. Here, direct, independent, and quantitative bulk probes of both cationic and lattice anionic redox chemistry are achieved through mapping of resonant inelastic X-ray scattering. We found highly reversible lattice oxygen redox in Na 2/3 Mg 1/3 Mn 2/3 O 2 (79%) and Li 1.17 Ni 0.21 Co 0.08 Mn 0.54 O 2 (76%) during the initial cycle, with 87% and 44% capacity retention after 100 and 500 cycles, respectively. Therefore, aside from other oxygen activities, lattice oxygen redox could be highly reversible in batteries.