Coverage dependence of oxygen adsorption on the hexagonal Co(11 ($) over bar 20) surface is studied at 100 and 320 K by means of Auger electron spectroscopy (AES), photoelectron spectroscopy (XPS, UPS) and change in work function (Delta phi). Adsorption of oxygen at 100 K leads to the formation of Co3O4-like features together with adsorbed oxygen. An influence of a precursor on the adsorption kinetics is proposed. Oxygen at 320 K builds up CoO, and the uptake behaviour is explained best in terms of chemisorption and oxide nucleation followed by island growth of the oxide. The explanation of the nature of a second oxygen species, observed in the Ols XP spectra, and the characterisation of OH are facilitated by a coadsorption experiment with oxygen and water at 100 K.