We experimentally explored adsorption and activation of O2 on small anionic clusters AuxOy- containing one to five gold atoms and between one and three oxygen atoms using an instrument including a magnetron sputtering cluster source, a micro flow reactor running at low temperature, and a time-of-flight mass spectrometer. Some species, including AuO-, one isomer of Au2O2-, Au3O-, one isomer of Au3O3-, and Au5O2-, can adsorb an O2 molecule. We theoretically explored the structures of these active species and the inert ones appearing in the experiment by combining a structure search strategy based on the genetic algorithm and the density functional theory (DFT) calculations. Impressively, all active species observed in the experiment have a -O-Au site, in which the gold atom is a dangling or a vertex atom. Each -O-Au site can strongly adsorb one O2 with its Au atom to form a straight-line structure -O-Au-O-, and the adsorbed O2 is significantly activated by accepting one electron with one of its π2p* orbitals. With no exception, all oxygen sites and the -O-Au-Au sites in AuxOy- are inert. Analyses on the density of states (DOS) of representative species well interpret the physical origins of the activity of -O-Au and the inertness of -O-Au-Au. The observations that site-specific factors dominate the reactivity of gold oxide clusters with O2 are in contrast to what happens in the reactions of Aun- with O2, where clusters' reactivity is completely determined by their global spins and electron detachment energies. The new conclusions in this work offer a reference to understand the crucial O2 activation processes in gold-based catalysts, since various gold oxide structures are commonly observed in these systems.