AbstractCeO₂ greatly enhances the electrocatalytic oxygen evolution reaction (OER) activity of CoO_x, though the enhancement mechanism beyond this synergy is yet to be understood. Here, operando hard X‐ray absorption spectroscopy (hXAS) is applied to monitor the Co K edge and Ce L₃ edge in CoO_x/CeO₂ to shed light on the evolution of the Co and Ce oxidation states during OER. In addition, ex situ soft XAS (sXAS) characterizations provide information on the irreversible surface‐specific transformations of the Co L₃ edge as well as of the O K edge. Combining the operando and ex situ spectroscopic characterizations with comprehensive electrochemical analyses, it is confirmed that CeO₂ is not the active center for the OER. However, coupling CeO₂ with CoO_x introduces significant modifications in the Co and O species at the CoO_x surface and alters the flat band potential (E_fb), leading to more favorable Co oxidation state transformations during OER and possibly modifying the preferential reaction pathway. This work establishes the connections between electronic structures, Co oxidation state and the OER reaction mechanism for CoO_x/CeO₂ composites electrodes.