Diazo compounds as coupling partners are efficiently applied to Ir(III)-catalyzed direct C–H functionalization of arylphosphine oxides. Involving C–H activation, carbene insertion, and tautomerism, this reaction proceeds under mild conditions, thus proving an approach to the synthesis of MOP-type ligand precursor in a single step. The utility of this transformation has been further demonstrated in ligand synthesis as well as in the construction of phosphole framework.