In the present work, photoinduced O-2 evolution from the [Cp2Os-OH](+) complex in aqueous solution has been studied by the DFT, CASSCF, and CASPT2 methods. The CASPT2//CASSCF calculations predict that the S-3 state is initially populated and the subsequent deprotonation of [Cp2Os-OH](+) proceeds very easily along the T-1 pathway as a result of the efficient S-3 -> T-1 intersystem crossing. It is found that the O-O bond is formed via the acid-base mechanism, which is different from the direct oxo-oxo coupling mechanism suggested in the experimental study. Formation of the O-O bond is the rate-determining step and has an . II activation energy and activation free energy of 81.3 and 90.4 kcal/mol, respectively. This is consistent with the low quantum yield observed for generating molecular oxygen upon irradiation at 350 nm (similar to 82 kcal/mol). The O-2 release from an intermediate complex has to overcome a small barrier on the triplet pathway first and then pass through the triplet-singlet the lowest singlet or triplet state. The formed O-3(2) molecule can be converted into the O-1(2) molecule by the heavy atom effect in the Os complexes, which is probably the reason only the O-1(2) molecule was detected experimentally.