We have investigated the interaction of oxygen with the Ag/Pt(111) surface alloy by thermal desorption spectroscopy (TDS). The surface alloy was formed during the deposition of sub-monolayer amounts of silver on Pt(111) at 800 K and subsequent cooling to 300 K. The low-temperature phase of the surface alloy is composed of nanometer-sized silver rich stripes, embedded within platinum-rich domains, which were characterized with spot profile analysis low energy electron diffraction. The TDS measurements show that oxygen adsorption is blocked on Ag sites: the saturation coverage of oxygen decreases with increasing Ag coverage. Also, the activation energy for desorption (E des ) decreases with Ag coverage. The analysis of the desorption spectra from clean Pt(111) shows a linear decay of E des with oxygen coverage, which indicates repulsive interactions between the adsorbed oxygen atoms. In contrast, adsorption on alloyed Ag/Pt(111) leads to an attractive interaction between adsorbed oxygen atoms.