The reactivity of oxygen adatoms formed by dissociative adsorption of O-2 on Cu(100) at 100 K for CO oxidation and hydrogen abstraction from water was investigated by means of HREELS and TPD in comparison with the reactivity of adsorbed oxygen in thermally stable phases. The oxygen adatoms formed on the Cu(100) surface by exposure to O-2 at 100 K, designated as as-exposed oxygen, were found to be reactive with coadsorbed CO to yield a CO2 desorption peak at 125 K in TPD. The as-exposed oxygen atoms are suggested to be more active for CO oxidation than the oxygen atoms prepared above room temperature on Pt and Pd, which are the most active metals for CO oxidation. On annealing the oxygen-as-exposed surfaces to 300 K, a change in the loss feature of nu(Cu-O) was observed, which was indicative of formations of a pseudo c(2 x 2)-O phase with O(a) in 4-fold hollow sites and a (root 2 x 2 root 2)R45 degrees-O phase comprised of -Cu-O- chains grown along the [001] direction. On these two phases, CO2 formation in TPD was suppressed by 1 order of magnitude. Isolated oxygen atoms in the as-exposed surface are responsible for the high reactivity for CO oxidation. Hydrogen abstraction from water was also examined as a probe reaction for oxygen adatoms on different phases. The as-exposed surface and the pseudo c(2 x 2)-O phase were found to be reactive and OH(a) was detected by means of HREELS after subsequent exposure to water at 100 K. In contrast, the (root 2 x 2 root 2)-R45 degrees-O phase in which oxygen atoms were incorporated into -Cu-O- chains was inert.