A new and promising P2-type layered oxide, Na5/6[Li1/4Mn3/4]O2 is prepared using a solid-state method. Detailed crystal structures of the sample are analyzed by synchrotron X-ray diffraction combined with high-resolution neutron diffraction. P2-type Na5/6[Li1/4Mn3/4]O2 consists of two MeO2 layers with partial in-plane √3a × √3a-type Li/Mn ordering. Na/Li ion-exchange in a molten salt results in a phase transition accompanied with glide of [Li1/4Mn3/4]O2 layers without the destruction of in-plane cation ordering. P2-type Na5/6[Li1/4Mn3/4]O2 translates into an O2-type layered structure with staking faults as the result of ion-exchange. Electrode performance of P2-type Na5/6[Li1/4Mn3/4]O2 and O2-type Lix[Li1/4Mn3/4]O2 is examined and compared in Na and Li cells, respectively. Both samples show large reversible capacity, ca. 200 mA h g−1, after charge to high voltage regardless of the difference in charge carriers. Structural analysis suggests that in-plane structural rearrangements, presumably associated with partial oxygen loss, occur in both samples after charge to a high-voltage region. Such structural activation process significantly influences electrode performance of the P2/O2-type phases, similar to O3-type Li2MnO3-based materials. Crystal structures, phase-transition mechanisms, and the possibility of the P2/O2-type phases as high-capacity and long-cycle-life electrode materials with the multi-functionality for both rechargeable Li/Na batteries are discussed in detail.