The efficient energy storage devices are required to overcome the problem of fluctuations in the availability of electric energy. The water electrolysis to produce storable hydrogen and oxygen is one of the most expecting approaches for large-scale energy storage. The proton exchange membrane water electrolysis (PEMWE) has a potential to produce the high purified hydrogen under the operation at high current densities. However, the reducing the Ir loading at the OER electrode is essential to realize large-scale applications of PEMWE. This requires the development of highly active OER catalysts to accelerate the reaction or reduce the overpotential is the key issues toward the widespread applications. Herein, we report a remarkably active OER catalyst of Ir oxide with nanorod-like structure supported on niobium-doped tin oxide (Nb-SnO2), showing an order of magnitude increase in Ir mass-specific activity than a commercial IrOx catalyst at 1.5 V vs. RHE. We also investigated the temperature dependence of the OER activity, as a way to elucidate the mechanism of enhanced catalysis, from both experimental and theoretical perspectives.