Iridium oxide-based catalysts are uniquely active and stable in the oxygen evolution reaction. Theoretical work attributes their activity to oxyl or μ1-O species. Verifying this intermediate experimentally has, however, been challenging. In the present study, these challenges were overcome by combining theory with new experimental strategies. Ab initio molecular dynamics of the solid–liquid interface were used to predict spectroscopic features, whereas sample architecture, developed for surface-sensitive X-ray spectroscopy of electrocatalysts in confined liquid, was used to search for these species under realistic conditions. Through this approach, we have identified μ1-O species during oxygen evolution. Potentiodynamic X-ray absorption additionally shows that these μ1-O species are created by electrochemical oxidation currents in a deprotonation reaction.