Titanium and zirconium oxide cluster anions with dimensions up to nanosize are prepared by laser ablation and reacted with carbon monoxide in a fast low reactor. The cluster reactions are characterized by time-of-flight mass spectrometry and density functional theory calculations. The oxygen atom transfers from (TiO2)(n)O- (n = 3-25) to CO and formations of (TiO2)(n)(-) are observed, whereas the reactions of (ZrO2)(n)O- (n = 3-25) with CO generate the CO addition products (ZrO2)(n)OCO-, which lose CO2 upon the collisions (studied for n = 3-9) with a crossed helium beam. The computational study indicates that the (MO2)(n)O- (M = Ti, Zr; n = 3-8) clusters are atomic radical anion (O-) bonded systems, and the energetics for CO oxidation by the O- radicals to form CO2 is strongly dependent on the metals as well as the cluster size for the titanium system. Atomic oxygen radical anions are important reactive intermediates, while it is difficult to capture and characterize them for condensed phase systems. The reactivity pattern of the O--bonded (TiO2)(n)O- and (ZrO2)(n)O- correlates very well with different behaviors of titania and zirconia supports in the low-temperature catalytic CO oxidation.