The study investigates the enhancement mechanism of the electrocatalytic oxygen evolution reaction (OER) activity of CoOₓ by CeO₂. Using operando hard X-ray absorption spectroscopy (hXAS) at the Co K edge and Ce L₃ edge, the researchers monitor the oxidation states of Co and Ce during OER. Ex situ soft XAS (sXAS) characterizations provide additional insights into the surface-specific transformations of Co and O species. The results show that CeO₂ is not the active center for OER, but its interaction with CoOₓ significantly modifies the Co and O species at the CoOₓ surface, altering the flat band potential (Eₕ) and favoring more favorable Co oxidation state transformations. This work establishes a connection between electronic structures, Co oxidation states, and the OER reaction mechanism for CoOₓ/CeO₂ composites. The study also reveals that the pH-dependence of OER activity indicates different reaction pathways for CoOₓ and CoOₓ/CeO₂, suggesting that CeO₂'s role is to modify the electronic structure and surface properties of CoOₓ, rather than directly participating in the OER reaction.The study investigates the enhancement mechanism of the electrocatalytic oxygen evolution reaction (OER) activity of CoOₓ by CeO₂. Using operando hard X-ray absorption spectroscopy (hXAS) at the Co K edge and Ce L₃ edge, the researchers monitor the oxidation states of Co and Ce during OER. Ex situ soft XAS (sXAS) characterizations provide additional insights into the surface-specific transformations of Co and O species. The results show that CeO₂ is not the active center for OER, but its interaction with CoOₓ significantly modifies the Co and O species at the CoOₓ surface, altering the flat band potential (Eₕ) and favoring more favorable Co oxidation state transformations. This work establishes a connection between electronic structures, Co oxidation states, and the OER reaction mechanism for CoOₓ/CeO₂ composites. The study also reveals that the pH-dependence of OER activity indicates different reaction pathways for CoOₓ and CoOₓ/CeO₂, suggesting that CeO₂'s role is to modify the electronic structure and surface properties of CoOₓ, rather than directly participating in the OER reaction.