Nickel is less commonly employed for O2 activation than iron or manganese, due to its relatively high effective nuclear charge. Thus, most cases require low-valent Ni(I) or Ni(0) centers, necessitating the use of additional reductants for O2 activation. However, O2-derived oxidation and oxygenation of divalent organonickel complexes are achievable, albeit often in an uncontrolled manner, implying the significance of carbon coordination in the reductive activation of O2. We have found that organonickel complexes exhibit lower reduction potentials than their inorganic counterparts due to the strong σ-donation of carbon ligands. This effect is sufficiently pronounced to enable both divalent and even trivalent organonickel complexes to reduce O2, demonstrating the σ non-innocence of the Ni-C bonds. The O2 activation pathways of organonickel complexes turned out to resemble those of mono- and binuclear non-Heme iron enzymes, with the mechanisms depending on the ancillary ligands and the initial formal oxidation state of the nickel center. Oxygenated intermediates have been kinetically isolated and characterized by resonance Raman and electronic absorption spectroscopies, revealing that the rate-determining intermediates of the binuclear pathways could be either bis(m-oxo) or peroxo-bridged species, while that of the mononuclear pathway was a peroxo species. Notably, the reactivity of the bis(m-oxo) species could be selectively directed toward either C-C or C-O bond formation depending on its electronic state. Additionally, the bis(m-oxo) bridge could be converted to a peroxo bridge upon further oxidation. While the oxo moiety readily facilitated the oxygenation of the carbon ligand, the peroxo species required an exogenous electron source for further activation to oxygenate the organic ligand. These findings demonstrate that the O2 reactivity of organonickel complexes parallels that of monooxygenase enzymes, highlighting how insights from nature can guide the development of selective O2 activation in organometallic chemistry.