A chromium(I) dinitrogen complex reacts rapidly with O2 to form the mononuclear dioxo complex TptBu,MeCrV(O)2 (TptBu,Me=hydrotris(3‐tert‐butyl‐5‐methylpyrazolyl)borate), whereas the analogous reaction with sulfur stops at the persulfido complex TptBu,MeCrIII(S2). The transformation of the putative peroxo intermediate TptBu,MeCrIII(O2) (S=3/2) into TptBu,MeCrV(O)2 (S=1/2) is spin‐forbidden. The minimum‐energy crossing point for the two potential energy surfaces has been identified. Although the dinuclear complex (TptBu,MeCr)2(μ‐O)2 exists, mechanistic experiments suggest that O2 activation occurs on a single metal center, by an oxidative addition on the quartet surface followed by crossover to the doublet surface. Forbidden or allowed? The four‐electron oxidative addition of O2 to TpCrI complexes (Tp=tris(pyrazolyl)borate) is spin‐forbidden, and yet, it proceeds by a non‐adiabatic crossing from a quartet to a doublet potential energy surface at the minimum‐energy crossing point, the structure of which is shown.