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 O2 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.