Among the candidate cathode materials for sodium-ion batteries (SIBs), Na_xMnO₂ (NMO) layered oxides are especially attractive in terms of working potential (ca. 3.0 V vs. Na⁺/Na) and high initial specific charge. In this work we discuss the influence of the sodium/manganese arrangements in P2 and P'2-type phases and P2/P'2 phase ratio on the electrochemical performance of NMO electrodes. The detailed structural characterization of the materials was achieved by X-ray diffraction (XRD), neutron powder diffraction (NPD), inductively coupled plasma (ICP), and scanning electron microscopy (SEM/EDS). By combining the results from X-ray absorption spectroscopy (XAS), operando XRD, and online electrochemical mass spectrometry (OEMS) techniques we found that it is possible to change the reaction mechanisms from the P2 + P'2-type → OP4-type phase (observed for biphasic P2/P'2 Na_xMnO₂) to an intergrowth mechanism (single P2 Na_xMnO₂) by tuning the pristine composition. Additionally, we further discuss the influence of the reaction mechanism on the electrochemical performance. Based on the standard cycling protocol and rate capability tests we found that appropriate synergy of both P2 and P'2 is essential for obtaining Na_xMnO₂ electrodes with high specific charge offering long-term cycling.