The pursuit of advanced battery chemistries with enhanced energy density necessitates the exploration of new materials, a process intricately tied to synthesis science. Despite the promise of O3-type sodium oxygen anionic redox cathodes as high-capacity materials, their development has been severely hindered by a lack of understanding regarding synthetic mechanisms. Here, we elucidate the pivotal role of atmospheric conditions, particularly oxygen content, in the synthesis of such materials by synchronizing multiple operando characterization techniques to monitor changes in both solid and gaseous components. Utilizing the O3-Na[Li