The need to store an ever increasing amount of renewable energy in a sustainable way has rekindled interest for Na-ion batteries owing to the abundance of Na. Their energy density, lower than Li-Ion, can be enhanced by unlocking anionic redox, as recently reported in Na-deficient P2-phases. In contrast to their Li-rich counterparts with O3 stacking, these Na-deficient P2-phases show surprisingly good structural stability during anionic redox. Understanding the fundamental science at work in the relation between O/P stacking and anionic redox reversibility is critical to design stable anionic redox cathodes. Herein, through DFT-based analysis of the model compounds O2-and P2-Na2∕ 3Mg1∕3Mn2∕3O2, we show that the anionic redox process corresponds to a highly reversible collective distortion of the oxygen network in P stacking, or to a disproportionation of the oxygen pairs associated with significant voltage hysteresis in O stacking. Based on these findings, we used a magnetic-constrained DFT methodology to quantitatively predict the composition range of the reversible cycling that we successfully extend to other Mn-based cathodes (Na2∕ 3Zn1∕3Mn2∕3O2, Na 2 Mn 3 O 7). This article thus provides fundamental understanding, powerful computational methods and practical guidelines to design more stable anionic redox compounds.