Na-ion batteries (NIBs) are increasingly of interest in modern society as cheaper energy storage alternatives to Li-ion batteries. At present, the capacity of NIBs is too low for practical applications and limited by the cathode. However, some Mn-based cathode materials have shown large capacities at voltages above the conventional Mn3+/4+ redox couple, attributed to reversible redox reactions involving O2−. Here, we propose to measure the O K-edge X-ray absorption spectra for two NIB cathodes, Na0.67[Mg0.28Mn0.72]O2 and Na4/7Mn6/7O2, at several states of charge and determine to what extent the oxidation state and coordination environment of O changes, particularly at high voltages. These results, alongside ex situ NMR, ESR and Mn K-edge XANES data, as well as periodic DFT calculations of the electronic structures of these materials, will be used to identify whether oxidised O species are generated, and if so, how they are stabilised by the chemical structure of the cathode.