We here precisely analyzed the electronic structures during the oxygen-oxygen (O-O) bond formation of the oxygen-evolving complex (OEC) in photosystem II (PSII) based on the quantum mechanics/molecular mechanics (QM/MM) calculations. Changes in the spin density and the chemical indices derived from the natural orbitals suggested that the O-(5)-O-(6) bond formation proceeds with a concerted bond breaking of the Mn-4-O-(5) coupled with the through-bond one-electron transfer from the O-(6) site to the Mn-4 site (Mn-4(IV) -> Mn-4(III)). This mechanism can be best described as a Concerted Bond Switching (CBS) mechanism, which is a nonradical reaction of the O-(5)-O-(6) bond formation.