Impingement of O-16-atoms on an O-18(2) covered Pt(111) surface at 77 K induces the evolution of gas-phase O-18(2) and (OO)-O-16-O-18 in a ratio of similar to 4:1 independent of atomic flux or kinetic energy. The total initial probability of desorprion of molecolar oxygen induced bl impingement of atomic oxygen is measured as a function of incident O-atom flux, kinetic energy, and angle. Interestingly, the probability is found to depend on incident angle with values ranging from 0.25 +/-0.02 for a 60 degrees angle of incidence to 0.42+/-0.02 for normal incidence.