The adsorption behavior of oxygen and iodine on a Cs-precovered Pt(111) surface is studied with Low Energy Electron Diffraction (LEED), Thermal Desorption Spectroscopy (TDS) and work function measurements. TDS shows multiple oxygen desorption states that depend on Cs coverage, with both atomic and molecular oxygen desorbing from the surface. Cs is stabilized by oxygen relative to Cs on Pt(111), and most is retained on the surface until the last oxygen desorbs. Atomic arrangements are proposed for two (4 × 4)-Cs,O structures observed by LEED, and these are similar to Cs,I structures with ionic character. At the highest oxygen and Cs coverages, there is evidence for a molecular oxygen state above room temperature. Dosing iodine on Pt(111)-Cs,O results in substitution of O by I in Cs,O islands. Oxygen adsorption on a Cs-saturated Cs monolayer at 295 K causes an initial work function decrease consistent with underlayer adsorption, i.e., with O adsorption below the plane of the Cs atoms. This feature is absent for adsorption on the lower-coverage Pt(111) (2 × 2)-Cs surface.