Recent advances of ambient pressure X-ray photoelectron spectroscopy (AP-XPS) have enabled the chemical composition and the electrical potential profile at a liquid/electrode interface under electrochemical reaction conditions to be directly probed. In this work, we apply this operando technique to study the surface chemical composition evolution on a Co metal electrode in 0.1 M KOH aqueous solution under various electrical biases. It is found that an similar to 12.2 nm-thick layer of Co(OH)(2) forms at a potential of about -0.4 V-Ag/AgCl, and upon increasing the anodic potential to about +0.4 V-Ag/Cl, this layer is partially oxidized into cobalt oxyhydroxide (CoOOH). A CoOOH/Co(OH)(2) mixture layer is formed on the top of the electrode surface. Finally, the oxidized surface layer can be reduced to Co-0 at a cathodic potential of -1.35 V-Ag/Cl. These observations indicate that the ultrathin layer containing cobalt oxyhydroxide is the active phase for oxygen evolution reaction (OER) on a Co electrode in an alkaline electrolyte, consistent with previous studies.