Hyperthermal collisions of ground-state atomic oxygen with H2O have been investigated, with special attention paid to the H-atom elimination reaction, O(P-3) + H2O(X-A1(1)) -> HO, ((2)A(1)) + H(S-2). This reaction was observed in a crossed-beams experiment, and the relative excitation function in the region around its energy threshold (50-80 kcal mol(-1)) was measured. Direct dynamics calculations were also performed at two levels of theory, B3LYP/6-31G(d,p) and MP2/6-31G(d,p). The shape of the B3LYP excitation function closely matches that of the experiment. The calculations provided a detailed description of the dynamics and revealed a striking dependence of the reaction mechanism on collision energy, where the cross section rises from a threshold near 60 kcal mol(-1) to a peak at -115 kcal mol(-1) and then decreases at higher energies as secondary dissociation of the internally excited HO2 product becomes dominant. The calculations show that the cross section for H-atom elimination (0 + H2O -> HO2 + H) is about 10-25% that of the H-atom abstraction (O + H2O -> OH + OH) cross section for collision energies in the 70-160 kcal mol(-1) range.