Li-rich Mn-based oxides (LRMO) are promising cathode materials to build next-generation lithium-ion batteries with high energy density exceeding 400 W h kg−1. However, due to a lack of in-depth understanding of oxygen redox chemistry in LRMO, voltage decay is not resolved thoroughly. Here, it is demonstrated that the oxygen redox chemistry could be tuned by modulating cation arrangement. It declares that the materials with Li/Ni disorder and Li vacancies can inhibit the formation of OO dimers. Because of the high chemical activity, OO dimers could accelerate lattice oxygen release and NiO/spinel formation. The samples without forming OO dimers show improved performance in suppressing oxygen overoxidation and mitigating cation dissolution. As a result, the optimized cathode exhibits a high capacity over 280 mA h g−1 at 0.1 C and a high plateau voltage of 3.58 V with a very low voltage decay of 1.6% after 150 cycles at 1 C. This study opens an attractive path in designing Li-rich electrodes with stabilized redox chemistry.