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Iridium oxide (IrOx) is one of the best catalysts for the aqueous oxygen evolution reaction (OER), and its activity is greatly impacted by surface characteristics. By reactively sputtering in a high O2flow-rate environment, vertically oriented IrOxnanoplatelets grow several hundred nanometers high exhibiting large surface areas and antireflective optical properties across the visible spectrum. The nanoplatelet IrOxsurface is electrochemically compared to other morphologies of IrOxsurfaces for OER activity. It was found that the nanoplatelet IrOxsurface outperforms all other tested morphologies and planar Ir-metal by exhibiting higher currents and lower overpotentials. Longevity testing of catalytic activity shows that the nanoplatelet surface is more stable in a wide pH range. Characterization with X-ray diffraction and X-ray photoelectron spectroscopy shows that the stoichiometry and oxidation states are similar between the different morphologies of IrOx, but the preferred crystallographic orientation of the rutile IrOxfilm changes at higher O2flow rates. The change from a (110) to a (101) growth direction corresponds with higher OER activity. Nanoplatelet IrOxfilms are therefore found to expose active sites preferable for the OER, and when combined with their antireflective properties these surfaces are promising for solar water-splitting applications.