Establishment of sustainable energy society has been strong driving force to develop cost-effective and highly active catalysts for energy conversion and storage devices such as metal-air batteries and electrochemical water splitting systems. This is because the oxygen evolution reaction (OER), a vital reaction for the operation, is substantially sluggish even with precious metalsbased catalysts. Here, we show for the first time that a hexagonal perovskite BaNiO3 can be highly functional catalyst for OER in alkaline media. We identify that the BaNiO3 performs OER activity at least an order of magnitude higher than an IrO2 catalyst. Using integrated density functional theory calculations and experimental validations we unveil the underlying mechanism is originatedfrom structural transformation from BaNiO3 to BaNi0.83O2.5 (Ba6Ni5O15) over the OER cycling process. Figure 1