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O2 formation in photosystem II (PSII) is a vital event on Earth, but the exact mechanism remains unclear. The presently prevailing theoretical model is "radical coupling"(RC) involving a Mn(IV)-oxyl unit in an "open-cubane"Mn4CaO6 cluster, which is supported experimentally by the S3 state of cyanobacterial PSII featuring an additional Mn-bound oxygenic ligand. However, it was recently proposed that the major structural form of the S3 state of higher plants lacks this extra ligand, and that the resulting S4 state would feature instead a penta-coordinate dangler Mn(V)=oxo, covalently linked to a "closed-cubane"Mn3CaO4 cluster. For this proposal, we explore here a large number of possible pathways of O-O bond formation and demonstrate that the "nucleophilic oxo-oxo coupling"(NOOC) between Mn(V)=oxo and μ3-oxo is the only eligible mechanism in such a system. The reaction is facilitated by a specific conformation of the cluster and concomitant water binding, which is delayed compared to the RC mechanism. An energetically feasible process is described starting from the valid S4 state through the sequential formation of peroxide and superoxide, followed by O2 release and a second water insertion. The newly found mechanism is consistent with available experimental thermodynamic and kinetic data and thus a viable alternative pathway for O2 formation in natural photosynthesis, in particular for higher plants.