The development of non-precious metal-based catalysts for efficient renewable energy conversion remains a significant challenge. In this study, we synthesized petal-shaped spherical electrocatalysts based on cobalt–nickel phosphate (NiCoP) doped with Mo at different concentrations (Mo-NiCoP; 0.05 %, and 0.1 %) using a facile process. Mo doping provides abundant active sites for electrochemical water oxidation. The 0.1 %Mo-NiCoP electrocatalyst exhibited superior electrocatalytic activity for the oxygen evolution reaction (OER), with a low overpotential of 277 mV, compared to NiCoP (313 mV), 0.05 %Mo-NiCoP (292 mV), and noble-metal catalysts for OER. Additionally, the long-term stability of 0.1 %Mo-NiCoP under a fixed voltage for 20 h revealed only a minor decline in current density. The results of density functional theory calculations suggest that the exceptional electrocatalytic activity of Mo-NiCoP is due to the optimized adsorption energy barrier of intermediates in the OER process, the modulation of the electronic structure, and the introduction of rich lattice disorder defects in NiCoP by the Mo heteroatoms. This work presents a practical and economical approach to fabricating ternary transition-metal phosphates for water oxidation.