Sodium-ion batteries are one of the most promising next-generation energy storage systems because of their abundant and low-cost component materials. However, the lower energy density of sodium-ion batteries compared with lithium-ion batteries diminishes their practical value proposition. Among the many sodium-based cathodes, layered transition metal oxides with high sodium content can achieve energy densities comparable with the lithium-ion battery technology. For some layered transition metal oxide cathodes, when they are charged to above 4 V, a long plateau is often observed before they reach to fully charged state, which involves structural transformations that lead to irreversible reactions. Apart from structural transformations, lattice oxygen anion redox has also been proposed as a mechanism from both experimental results such as X-ray absorption spectroscopy, resonant inelastic X-ray scattering studies and simulation efforts. Reversible lattice oxygen anion redox is often accompanied by the generation of peroxo (O₂^2-) or superoxo (O^2-) related species. Irreversible lattice oxygen anion redox is often accompanied by the generation of molecular O₂, along with transition metal ion migration, lattice distortion, and rapid capacity decay. In this work, in-situ gas analysis was performed to evaluate the gas generation in Na_xNi_yMn_1-yO₂ cathodes synthesized by eutectic method in sodium half-cell configuration. Operando x-ray diffraction and neutron diffraction were performed to assess the structural changes related to the high voltage plateau and transition metal ion migration in Na_xNi_yMn_1-yO₂ cathodes. The results unveil that in-plane honeycomb cationic ordering can help reduce the activity of irreversible oxygen anion redox which is critical for future design of layered transition metal oxide cathode that are prone to achieve high-energy for durable sodium-ion batteries.