The OP4-(Li/Na)(x)CoO2 phase is an unusual lamellar oxide with a 1:1 alternation between Li and Na interslab spaces. In order to probe the loca structure, electronic structure, and dynamics, Li-7 and Na-23 magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy was performed in complementarity to X-ray diffraction and electronic and magnetic properties measurements. Li-7 MAS NMR showed that NMR shifts result from two contributions: the Fermi contact and the Knight shifts due to the presence of both localized and delocalized electrons, which is really unusual. Li-7 MAS NMR clearly shows several Li environments, indicating that, moreover, Co ions with different local electronic structures are formed, probably due to the arrangement of the Na+ ions in the next cationic layer. Na-23 MAS NMR showed that some Na(+ )ions are located in the Li layer, which was not previously considered in the structural model. The Rietveld refinement of the synchrotron XRD led to the OP4-[Li-0.42 Na-0.05]Na(0. 32 )CoO(2 )formula for the material. In addition, Li-7 and Na-23 MAS NMR spectroscopies provide information about the cationic mobility in the material: Whereas no exchange is observed for Li-7 up to 450 K, the( 23)Na spectrum already reveals a single average signal at room temperature due to a much larger ionic mobility.