Transition metal oxides present a promising alternative to noble metals for oxygen electrocatalysis, including cobalt-containing perovskite oxides for the oxygen evolution reaction (OER) in alkaline electrolyzers. However, a lack of fundamental understanding of oxide surfaces impedes rational catalyst design for improved activity and stability. Investigation of epitaxial oxide thin films allows quantification of the intrinsic activity, as well as examination of their chemical speciation in an aqueous environment using ambient pressure X-ray photoelectron spectroscopy (AP-XPS). We couple electrochemical studies of epitaxial La_1-xSr_xCoO_3-δ films with in situ and operando AP-XPS to investigate the surface chemistry and electronic structure. By quantifying the formation of hydroxyl (OH) groups in situ, we compare the relative affinity of different surfaces for this key reaction intermediate in oxygen electrocatalysis. Operando measurements of La_0.8Sr_0.2CoO_3-δ in alkaline electrolyte indicate that the surface is stable during OER and behaves as a metal, with the voltage drop confined to the electrolyte. Furthermore, OH and carbonate species persist on the electrode surface even under oxidizing conditions, and may impact the availability of active sites or the binding strength of adsorbed intermediates via adsorbate-adsorbate interactions. The accumulation of OH species with oxidative potentials suggests that, for covalent oxides with facile charge transfer kinetics, the rate of reaction could be limited by proton transfer kinetics. This operando insight highlights the potential importance of carbonates in oxygen electrocatalysis and will help guide modeling of self-consistent oxide electrocatalysts.