Gold nanoparticles supported on reducible metal oxides have a high catalytic activity for CO oxidation at low temperatures, and there is great interest in further developing such catalysts and extending the utility of these catalysts for a wide range of selective hydrocarbon oxidation reactions. One aspect of improving our fundamental understanding of these reactions is further investigations of the reactivity of oxygen adatoms on well-defined gold surfaces. Oxygen adatoms on Au single crystal surfaces were generated cleanly under UHV conditions by exposure to ozone. Reactivity of this oxygen was probed with CO and several alkene oxidation reactions. Oxygen adatoms on a stepped Au(211) surface readily oxidize CO at 85 - 450 K, with an apparent activation energy of -7 kJ/mol. This is similar to that on flat Au(111) surfaces. In addition, infrared reflection-absorption spectroscopy (IRAS) identified CO species adsorbed on the O-precovered Au(211) surface that indicate the existence of positively charged Au sites, even on this bulk Au sample. In other studies, oxygen adatoms on Au(111) show a high activity for alkene oxidation. Reaction of propene, 1-butene, and 2,3-dimethyl-2-butene on this surface mainly produced CO2, H2O and epoxides at low temperature.