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In water splitting, anode catalysts for the oxygen evolution reaction (OER), which is the rate‐determining step, are more critical than cathode catalysts. Herein, the authors prepare Ni‐IrOx composite catalysts consisting of NiO and IrO2 for the OER by a solid‐state reaction with different ratios of NiO to IrO2 and reaction temperatures. In particular, Ni‐IrOx‐400 with a molar ratio of NiO/IrO2 = 1:1 heated at 400 °C shows the best OER performance. In the overall water splitting test using an anion exchange membrane (AEM) water electrolyzer, the single cell with Ni‐IrOx‐400 as the anode catalyst shows current densities of 1454.8 mA cm−2, respectively, measured at 1.8 V. Furthermore, the stability tests of the AEM single cells are carried out at 50 °C under a constant current density of 500 mA cm−2. The single cell with Ni‐IrOx‐400 shows only a slight increase in the overpotential (rate: 2.0 mV h−1) for 100 h owing to the enhanced stability of Ni‐IrOx‐400 compared to IrO2 (12.5 mV h−1). The improved OER performance of the Ni‐IrOx‐400 may be attributed to a composite structure that can prevent particle agglomeration and thus preserve the active sites during the OER. [ABSTRACT FROM AUTHOR]