To dispose of atomic oxygen, it is necessary the O-2 activation; however, an energy barrier must be overcome to break the O-O bond. This work presents theoretical calculations of the O-2 adsorption and dissociation on small pure Au-n and Ag-m and bimetallic AunAgm (n + m <= 6) clusters using the density functional theory (DFT) and the zeroth-order regular approximation (ZORA) to explicitly include scalar relativistic effects. The most stable AunAgm clusters contain a higher concentration of Au with Ag atoms located in the center of the cluster. The O-2 adsorption energy on pure and bimetallic clusters and the ensuing geometries depend on the spin multiplicity of the system. For a doublet multiplicity, O-2 is adsorbed in a bridge configuration, whereas for a triplet only one O-metal bond is formed. The charge transfer from metal toward O-2 occupies the sigma*(O-O) antibonding natural bond orbital, which weakens the oxygen bond. The Au-3 ((2)A) cluster presents the lowest activation energy to dissociate O-2, whereas the opposite applies to the AuAg ((3)A) system. In the O-2 activation, bimetallic clusters are not as active as pure Au-n clusters due to the charge donated by Ag atoms being shared between O-2 and Au atoms.