The formation of mono- and dinitrosyl species on highly oxidized Mo(110) surfaces is investigated as a model for NO reduction induced by metal oxides. Defects and oxygen vacancies are shown to determine the mechanism for dinitrosyl formation and subsequent reduction to N2O. On a highly defective oxide surface prepared by oxidation at 1200 K, mononitrosyl species are exclusively detected at low NO coverage using reflection absorption infrared spectroscopy (RAIRS). No low-temperature reduction of NO is observed under these conditions: instead, NO desorbs below 300 K. A dinitrosyl species, i.e., a species where two NO molecules are bound to the same metal center, is formed at higher coverages on the defective oxide. Notably, the dinitrosyl is not formed by addition to mononitrosyl species. The low temperature reduction of NO to N2O occurs only when the dinitrosyl is present. On a less defective thin-film oxide prepared at a surface temperature of 800 K, an NO overlayer consisting almost exclusively of dinitrosyls is formed at saturation coverage. These dinitrosyl species undergo competing reactions: reduction to N2O and decoupling to gaseous NO. Infrared spectroscopy is used to show that monomeric NO and the dinitrosyl species occupy different sites on the defective thin-film oxide on the basis of changes in the Mo=O stretch region of the spectrum. The changes in the Mo=O stretch intensities are attributed to the displacement of specific types of terminal oxygen (at steps and on terraces) by NO. These results indicate that NO creates its own adsorption sites. This characteristic is probably related to the defect density of the oxide.