Determining the relationships between structure, oxidation state and oxygen evolution reaction (OER) activity is fundamental to design improved anode catalysts for polymer electrolyte membrane water electrolyser (PEMWE) devices. In this study, four IrOx nanocatalysts differing from each other by the chemical composition were characterized before and after ageing in conditions simulating PEMWE anode operation. By combining identical location transmission electron microscopy (IL-TEM), X-ray photoelectron spectroscopy and electrochemistry, we highlighted and decoded complex relationships between chemical composition of the different nanocatalysts, nature of the support material, and OER electrocatalytic activity at the beginning and the end of life of the electrode. Independently on the chemical nature of the fresh nanocatalyst, Ir(III), Ir(IV) and Ir(V) species were monitored after electrochemical ageing. Our analysis suggests that Ir(V) species account for a major part of the OER activity, while Ir(III) species are responsible for its decay.