Lattice anionic (oxygen) redox reactions (ARR) offers opportunities for developing high-capacity batteries, however, often suffers the notoriously high hysteresis in voltages and low initial coulombic efficiency. Particularly, ARR was widely considered inherent to these kinetic issues. In this paper, unambiguous evidences of strong and reversible ARR is found in Na2/3Ni1/3Mn2/3O2 through mapping of resonant inelastic X-ray scattering (mRIXS). Strikingly, the material displays negligible voltage hysteresis (0.1 V) and high initial coulombic efficiency with a highly stable electrochemical profile. Our independent analysis of the Ni, Mn and O states consistently interpret the redox mechanism of Na2/3Ni1/3Mn2/3O2, which reveals a strong ARR system with facile kinetics and highly stable electrochemical profile that previously found only in cationic redox systems of conventional non-Alkali-rich materials.