The recent development and the in-depth analysis of O3-phase Na2/3Fe2/3Mn1/3O2 has enriched the understanding of the overall electrochemical performance relating to the phase transitions during cycling of layered oxide cathodes. In order to understand the correlation between the initial polymorphs, their phase transitions during cycling without bias from starting Na concentration, we directly compare the structural evolution of O3- and P2-phase Na2/3Fe2/3Mn1/3O2 utilizing time-resolved in situ synchrotron X-ray diffraction data. The complex phase transitions along with multiple regions of solid solution and biphasic evolution are identified based on time (or voltage) of cycling progress. The focus of this work is at relatively high C-rates of 1C and C/2.5, where a direct comparison of the evolution of the O3 and P2 type Na2/3Fe2/3Mn1/3O2 phases are made, showing striking differences which do have ramifications on the short and long-term electrochemical performance of these electrodes. An example of the mid-term structural evolution of the O3 electrode shows that it can transform to the P2 electrode with electrochemical cycling.