The structure of oxygen adsorbed on Ir{100} has been determined by density functional theory slab calculations and quantitative low energy electron diffraction. The two studies produce essentially identical results, experiment providing a benchmark for the first principles calculation, while the latter provides a more complete understanding of the structure. Oxygen forms a (1 x 2) overlayer with adsorption on the bridge site: an unusual result for oxygen. There is a significant row pairing of the iridium atoms in the surface layer which strongly stabilizes the structure, and is one of the main factors favoring the bridge site occupation and the (1 x 2) configuration.