A molecular oxygen beam has been used to study the dissociative and molecular sticking probability at the Ag(111) surface as a function of incidence energy and angle, surface temperature and surface coverage. Two mechanisms are seen to play a role in the dissociative sticking: a direct and a precursor mediated one. Both are strongly activated by the incidence energy. The molecular chemisorption state is shown to be the precursor to dissociation. Results on oxygen desorption are also discussed.