The interaction of oxygen and potassium on MgO(100) has been studied using thermal desorption spectroscopy (TDS) and X-ray photoelectron spectroscopy (XPS). The results show that several oxygen species are present on the surface. An atomic oxygen species is initially present at lower θ K as shown by the evidence of O 1s BE at 528.5 eV, together with the consistent but not coincident desorption of potassium and oxygen at 610 and 525 K, respectively. The oxygen desorption at a low temperature of 425 K coupled with the O 1s BE at 532.1 eV indicates the associative molecular oxygen adsorption on oxygen vacancies in the substrate. The non-stoichiometric 3D K O clusters form on the surfaces for θ K =< 1 ML. The desorption of K 2 O 2 followed by K 2 O at ~600 K was observed.