The reaction of 10-60 eV O+ ions with a silicon oxide thin film produces scattered O-2(-). Isotopic labeling experiments demonstrate that the O-2(-) product is formed by an abstraction reaction and not by physical sputtering. Energy and angle resolved detection reveals a correlation between the scattered and incident particle momenta, indicative of a direct process in which the incoming oxygen atom reacts with an adsorbed oxygen atom through an Eley-Rideal mechanism.