Dioxygen O-O bond activation is a process for oxygenases and oxidases to perform biological functions and synthetic biomimetic catalysts to carry out oxygenation reactions using molecular O-2 as an oxidant. Inspired by the experimental development of a Co-III -peroxo complex (i.e., [Co-III (TBDAP)(O-2)](+), TBDAP = N,N-ditert-butyl-2,11-diaza[3.3](2,6)-pyridinophane) that exhibits dioxygenase-like reactivity to activate nitriles, a density functional theory (DFT) mechanistic study has been carried out to understand how the peroxo ligand is broken to activate nitriles. The study unveils that the O-O bond cleavage takes place via conversion to a Co-II-superoxo complex aided by nitrile coordination, followed by formation of a five-membered intermediate via superoxo O-2 radical nucleophilic attack at the nitrile carbon. Finally, a [1,3]-sigmatropic rearrangement-like process breaks the dioxygen bond. The otherwise difficult [1,3]-sigmatropic rearrangement is enabled by the mediation of Co-III (TBDAP) which alters a concerted rearrangement to a sequential process of O-O bond cleavage and N-O bond formation. Expectedly, the unveiling of the O-O bond cleavage mechanism could offer a clue for the development of biomimetic metal oxygenation catalysts.