Co–Mn spinel oxide is a promising next-generation electrocatalyst that has previously shown oxygen reduction reaction activity that rivals that of Pt in alkaline fuel cells. Although the performance is encouraging, understanding the catalytic mechanisms in the oxygen reduction reaction is critical to advancing and enabling low-cost alkaline fuel cell technology. Here we use multimodal in situ synchrotron X-ray diffraction and resonant elastic X-ray scattering to investigate the interplay between the structure and oxidation state of a Co–Mn spinel oxide electrocatalyst. We show that the Co–Mn spinel oxide electrocatalyst exhibits a kinetically limited cubic-to-tetragonal phase transition, which is correlated to a reduction in both the Co and Mn valence states. Additionally, the electrocatalyst exhibits a reversible and rapid increase in tensile strain at low potentials during cyclic voltammetry, and joint density-functional theory is used to provide insight into how reactive adsorbates induce strain in spinel oxide nanoparticles.