•Experimental results on the natural water oxidation catalyst are summarized.•Theoretical studies on the natural water oxidation catalyst are summarized and compared with the experimental results.•Comparisons between experimental and theoretical studies point to a unified mechanism for natural water oxidation.•The catalyst serves as a blueprint for artificial catalyst of water oxidation. The aim of this review is to elucidate geometric structures of the catalytic CaMn4Ox (x = 5, 6) cluster in the Kok cycle for water oxidation in the oxygen evolving complex (OEC) of photosystem II (PSII) based on the high-resolution (HR) X-ray diffraction (XRD) and serial femtosecond crystallography (SFX) experiments using the X-ray free-electron laser (XFEL). Quantum mechanics (QM) and QM/molecular mechanics (MM) computations are performed to elucidate the electronic and spin structures of the CaMn4Ox (x = 5, 6) cluster in five states Si (i = 0 ∼ 4) on the basis of the X-ray spectroscopy, electron paramagnetic resonance (EPR) and related experiments. Interplay between the experiments and theoretical computations has been effective to elucidate the coordination structures of the CaMn4Ox (x = 5, 6) cluster ligated by amino acid residues of the protein matrix of PSII, valence states of the four Mn ions and total spin states by their exchange-couplings, and proton-shifted isomers of the CaMn4Ox (x = 5, 6) cluster. The HR XRD and SFX XFEL experiments have also elucidated the biomolecular systems structure of OEC of PSII and the hydrogen bonding networks consisting of water molecules, chloride anions, etc., for water inlet and proton release pathways in PSII. Large-scale QM/MM computations have been performed for elucidation of the hydrogen bonding distances and angles by adding invisible hydrogen atoms to the HR XRD structure. Full geometry optimizations by the QM and QM/MM methods have been effective for elucidation of the molecular systems structure around the CaMn4Ox (x = 5, 6) cluster in OEC. DLPNO-CCSD(T0) method has been applied to elucidate relative energies of possible intermediates in each state of the Kok cycle for water oxidation. Implications of these results are discussed in relation to the blueprint for developments of artificial catalysts for water oxidation.