Sodium-ion battery technology has attracted significant attention due to its substantial cost advantage and similar operating mechanism to Li-ion batteries. P2-type sodium manganese oxide cathode is one of the most promising candidates, demonstrating both high capacity and good cycling stability. Here, we explore the lattice oxygen activity in layered sodium transition metal oxides. We synthesize a series of sodium lithium manganese oxides, NaxLi0.25Mn0.75O2 (x = 0.75 - 0.833), to optimize Na content. We further investigate the charge compensation mechanism for the best performing Na0.75Li0.25Mn0.75O2 over an extensive electrochemical cycling window. The large charge and discharge capacity is enabled by reversible lattice oxygen redox in the high voltage region (>= 2.5 V), along with Mn redox at the voltages below 2.5 V. Additionally, we reveal a small amount of oxygen gas evolution, 0.04% of the total oxygen in Na0.25Li0.25Mn0.75O2. This initial study will trigger an interest in the lattice oxygen activity in layered sodium metal oxide cathode, therefore, leading to better understanding of its correlation with crystal structure and electrochemical performance. (c) The Author(s) 2019. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited.