The anionic redox reaction (ARR) can be used to enhance the capacity of cathode materials. Understanding how ARR proceeds along with transition metal redox electrochemistry is crucial for designing the next-generation high-energy-density cathodes. In this study, we use a Na2/3Mg1/3Mn2/3O2 (NMMO) thin film as a model cathode material for Na-ion batteries to study the oxygen redox reaction. We are able to control the orientation of the thin film using a pre-conditioning step of the Pt current collector layer and, thus, tune the cathode electrolyte interphase (CEI). Preferential orientation along the (001) direction prevents the extraction of Na+ ions from the lattice during charge. However, the randomly oriented NMMO thin films perform similar to the slurry electrode from powder. The characteristics of the electrode surface from XPS illustrate the evolution of CEI at different states of charge in the randomly oriented thin-film electrode. Our results show that the crystalline orientation can drastically affect the availability of the electrochemical reaction.