Molecular beam techniques have been used to measure the dissociative sticking probability of oxygen on Ag(110) as a function of the surface oxygen coverage. An initial rapid uptake is found to be followed by a region of much slower adsorption. The transition between these two regimes occurs upon formation of the first extended AgO added row structures and an explanation is given in agreement with models arising from STM studies. The reflectivity of the surface to helium decreases smoothly with oxygen coverage to a value characteristic of the oxygen adlayer.