An oxidation process in which a metal nitro complex transfers an oxygen atom from the nitro ligand to a substrate; especially such process conducted cyclically or catalytically using molecular oxygen as the oxygen source. Metal nitrosyl complex is formed as a coproduct together with an oxidation product of the substrate. In a cyclic process, nitrosyl ligand of the metal nitrosyl coproduct is reoxidized by molecular oxygen in presence of a monodentate base such as pyridine to nitro ligand; and the nitro complex, thus regenerated, can be used again to oxidize the substrate. In a catalytic process, using an activator such as a Lewis acid with alcohols or divalent palladium with olefins, elemental oxygen maintains a concentration of nitro ligand in the reaction mixture. In particular the metal is a Group VIII metal, especially cobalt and the ligand is saloph or tetraphenylporphyrin.