The chemisorption of molecular oxygen on the missing-row reconstructed Pt {110}( 1 x 2) surface has been investigated using ab initio calculations based on spin-density functional theory. The calculated energetic, structural, vibrational, and electronic properties of the chemisorbed O-2 species are discussed in terms of the available experimental data. We find that adsorption in the ridge-bridge site is strongly preferred on energetic grounds, relative to adsorption on the {111} microfacets or in the valley sites of the reconstructed surface. We argue in favor of initial adsorption in the ridge-bridge sites, followed by adsorption in the valley long-bridge sites at higher coverages, without excluding the possibility of bridge-site adsorption on the {111} microfacets.