The rational design of efficient catalysts for electrochemical water oxidation highly depends on the understanding of reaction pathways, which still remains a challenge. Herein, mononuclear and binuclear cobalt phthalocyanine (mono-CoPc and bi-CoPc) with a well-defined molecular structure are selected as model electrocatalysts to study the water oxidation mechanism. We found that bi-CoPc on a carbon support (bi-CoPc/carbon) shows an overpotential of 357 mV at 10 mA cm⁻², much lower than that of mono-CoPc/carbon (>450 mV). Kinetic analysis reveals that the rate-determining step (RDS) of the oxygen evolution reaction (OER) over both electrocatalysts is a nucleophilic attack process involving a hydroxy anion (OH⁻). However, the substrate nucleophilically attacked by OH⁻ for bi-CoPc is the phthalocyanine cation-radical species (Co^II–Pc–Pc˙⁺–Co^II–OH) that is formed from the oxidation of the phthalocyanine ring, while cobalt oxidized species (Pc–Co^III–OH) is involved in mono-CoPc as evidenced by the operando UV-vis spectroelectrochemistry technique. DFT calculations show that the reaction barrier for the nucleophilic attack of OH⁻ on Co^II–Pc–Pc˙⁺–Co^II–OH is 1.67 eV, lower than that of mono-CoPc with Pc–Co^III–OH nucleophilically attacked by OH⁻ (1.78 eV). The good agreement between the experimental and theoretical results suggests that bi-CoPc can effectively stabilize the accumulated oxidative charges in the phthalocyanine ring, and is thus bestowed with a higher OER performance.