The need to increase the energy production from renewable sources and their intermittent nature currently boost the development of energy storage systems such as proton exchange membrane water electrolysers (PEMWEs)[1]. These systems are currently perceived as one of the most promising technology currently available. However, the sluggish oxygen evolution reaction (OER, anodic reaction) kinetics and the poor stability of the anodic material still limit their widespread development[2]. Iridium oxides (IrO2) are ones of the most active electrocatalysts for the OER[3], able to maintain high OER kinetics over the long term in the harsh operating conditions of a PEMWE anode[4]. Considering the low abundance of iridium on the Earth's crust and its high cost, designing and synthesizing tailored OER nanocatalysts is an essential step. Switching from bulk surfaces to nanocatalysts requires investigations at atomically smooth model surfaces such as single crystals[5]. The effect of different crystal facets of Ir electrodes (Ir(111), Ir(210) and nano-faceted Ir(210)) on the electro-oxidation and on the catalytic performance for the OER were compared. Atomic Force Microscopy (AFM) and Scanning Tunneling Microscopy (STM) were used to correlate the OER performance to the surface morphology. The open structure was found to be the most active, with improved performance upon electro-oxidation. This study improves the comprehension of iridium electrocatalytic behavior upon oxidation and OER, and will help to design efficient catalysts for PEMWEs.