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O-redox in compounds with Li on the transition-metal layers (TML) has recently been attributed to the formation of molecular O₂ on charge, trapped in the lattice. Here, we show that a similar process occurs for P2-Na_0.67[Mn_0.72Mg_0.28]O₂, which contains Mg²⁺ on the TML. The molecular O₂ is identified by high-resolution RIXS and quantified by magnetometry, showing that it equates to the charge passed. This O₂ is trapped in voids that are formed by Mg²⁺ out-of-plane displacement and Mn⁴⁺ in-plane disordering and is then reduced on discharge associated with a large voltage hysteresis. In contrast to compounds containing Li⁺ in the TML, in which the honeycomb ordering and the high-voltage plateau are irreversibly lost after the first cycle, in P2-Na_0.67[Mn_0.72Mg_0.28]O₂, the plateau reappears partially on the second charge due to the partial reversibility of Mn in-plane and Mg out-of-plane migration and the …