The catalytic reaction between adsorbed oxygen atoms and CO molecules on Pt(111) was investigated by scanning tunneling microscopy and modeled by Monte Carlo simulations. Experiments were performed by dosing preadsorbed O-ad layers with CO between 237 and 274 K. Two stages were observed during dosing with CO, an initial reordering and compression of (2x2)O-ad islands, and a subsequent shrinking of the islands by the reaction of O-ad to give CO2. The reaction occurs exclusively at boundaries between (2x2)O-ad and c(4x2)COad domains. The reaction order with respect to the oxygen coverage is 0.5; the reactivity of the boundary increases during the reaction. The Monte Carlo simulations included surface diffusion of O-ad atoms, attractive interactions between O-ad atoms, the O-ad-COad reaction probability (with parameters from quantitative scanning tunneling microscopy measurements), adsorption/desorption of CO, and a high mobility of COad. The experimentally observed domain shapes, the reaction order of 0.5, and the increasing boundary reactivity could only be reproduced by additionally including an O-ad coordination-dependent activation energy DeltaE(react)* of 25 meV per O-ad neighbor that accounts for the attractive O-ad-O-ad interactions. The initial ordering stage could be modeled by incorporating an additional repulsive interaction between O-ad and COad. The fact that no reaction occurs in the interior of the (2x2)O-ad domains, although they are covered by a layer of interstitial COad molecules, is attributed to the crucial role of reactive O-ad-COad configurations that only exist at the domain boundaries.