Layered cobalt oxides are promising cathode materials for sodium ion secondary batteries (SIBs). By combined study of the X-ray absorption spectroscopy (XAS) around the O K-edge and ab initio calculation, we investigated the electronic state of the Na x CoO2 with different oxidization state, i.e, in O3-Na0.91CoO2 (CoO2 −0.91) and P2-Na0.66CoO2 (CoO2 −0.66). The O K-edge spectra in the pre-edge (529–536 eV) region shows significant change with oxidization of Na x CoO2. In O3-Na0.91CoO2, the spectra shows an intense band (B band) at 531 eV. In P2-Na0.66CoO2, the spectral weight of the B band increases and a new band (A band) appears at 530 eV. These spectral changes are qualitatively reproduced by the calculated partial density of states (pDOSs) of O3-NaCoO2 and P2-Na1/2CoO2. These results indicate that the electrons are partially removed from the O 2p state with oxidization of Na x CoO2.