We describe mechanistic studies of a C-H/C-O biaryl coupling of 1,3-azoles and aryl pivalates catalyzed by Ni(cod)2/dcype. This study not only supports a catalytic cycle consisting of C-O oxidative addition, C-H nickelation, and reductive elimination but also provides insight into the dramatic ligand effect in C-H/C-O coupling. We have achieved the first synthesis, isolation and structure elucidation of an arylnickel(II) pivalate, which is an intermediate in the catalytic cycle after oxidative addition of a C-O bond. Furthermore, kinetic studies and kinetic isotope effect investigations reveal that the C-H nickelation is the turnover-limiting step in the catalytic cycle.