The presence of chemisorbed oxygen on the Cu(111) surface is known to strongly reduce the activation barrier for water dissociation as compared to bare Cu(111). Here, we present direct experimental evidence for the hydrogen abstraction mechanism responsible for the facile H2O dissociation on an O/Cu(111) surface using reflection absorption infrared spectroscopy (RAIRS) in combination with isotopically labeled reactants. We also observe that chemisorbed hydroxyl species produced by water dissociation on the O/Cu(111) surface undergo an efficient hydrogen atom transfer from trapped water molecules, leading to the rapid replacement of the initial oxygen isotope coverage and the detection of only a single hydroxyl isotopologue on the surface, in apparent contradiction with the hydrogen abstraction mechanism. In the presence of Cu2O oxide islands on the O/Cu(111) surface, water dissociation occurs selectively at the edges of those islands, leading to the self-assembly of isotopically ordered structures.