The use of renewable electricity to prepare materials and fuels from abundant molecules offers a tantalizing opportunity to address concerns over energy and materials sustainability. The oxygen evolution reaction (OER) is integral to nearly all material and fuel electrosyntheses. However, very little is known about the structural evolution of the OER electrocatalyst, especially the amorphous layer that forms from the crystalline structure. Here, we investigate the interfacial transformation of the SrIrO3OER electrocatalyst. The SrIrO3amorphization is initiated by the lattice oxygen redox, a step that allows Sr2+to diffuse and O2-to reorganize the SrIrO3structure. This activation turns SrIrO3into a highly disordered Ir octahedral network with Ir square-planar motif. The final SryIrOx exhibits a greater degree of disorder than IrOx made from other processing methods. Our results demonstrate that the structural reorganization facilitated by coupled ionic diffusions is essential to the disordered structure of the SrIrO3electrocatalyst. © 2021 American Association for the Advancement of Science. All rights reserved.