Ni-catalysed functionalization of strong sigma C–O bonds has become an innovative alternative for forging C–C bonds from simple and readily available phenol-derived precursors. However, these methodologies are poorly understood in mechanistic terms. Here we provide mechanistic knowledge about how Ni catalysts enable sp²–sp² bond formation between aryl esters and arylzinc species by providing reliable access to on-cycle mononuclear oxidative addition species of aryl esters to Ni(0) complexes bearing monodentate phosphines with either κ¹– or κ²-O binding modes. While studying the reactivity and decomposition pathways of these complexes, we have unravelled an intriguing dichotomy exerted by Zn(II) salts that results in parasitic ligand scavenging, oxidation events and NiZn clusters. We provide evidence that coordinating solvents and ligands disrupt these processes, thus offering knowledge for designing more-efficient Ni-catalysed reactions and a useful entry point to unravel the mechanistic intricacies of related processes.