Electrochemical generation of oxygen via the oxygen evolution reaction (OER) is a key enabling step for many air-breathing electrochemical energy storage devices. IrO2 (Ir4+: 5d5) ranks among the most active known OER catalysts. However, it is unclear how the environment of the Ir4+ oxygen-coordination octahedra affects the OER electrocatalysis. Herein, we present the OER kinetics on a single-crystal, epitaxial SrIrO3(100)p perovskite oxide synthesized using molecular-beam epitaxy on a DyScO3(110) substrate. We find that by switching the host structure of the Ir4+ oxygen-coordination octahedra from corner- and edge-sharing rutile (IrO2) to purely corner-sharing perovskite (SrIrO3), the OER activity increases by more than an order of magnitude. We explain our finding with the correlated, semimetal electronic structure of SrIrO3; our density functional theory calculations reveal that the adsorption energetics on SrIrO3 depends sensitively on the electron-electron interaction, whereas for IrO2, it depends rather weakly. This finding suggests the importance of correlations on the OER and the design of future transition metal oxide electrocatalysts. © 2016 The Royal Society of Chemistry.