The interaction of gas-phase atomic oxygen with chemisorbed deuterium on Ru(001) has been investigated by means of temperature-programmed desorption (TPD) and high-resolution electron energy loss spectroscopy (HREELS). Exposure of gas-phase atomic oxygen to the; p(1x1) deuterium overlayer at a surface temperature of 80 K results primarily in the adsorption of oxygen atoms. Thermal desorption spectra measured after large atomic oxygen exposures show the desorption of D2O between 150 and 190 K as well as a large decrease in the activation energy for the recombinative desorption of deuterium. HREEL spectra demonstrate the presence of D2O following atomic oxygen exposure at 80 K and, together with TPD spectra, suggest the presence of chemisorbed OD. Since OD and D2O are formed at low surface temperatures, an Eley-Rideal-like mechanism is suggested in which oxygen atoms from the gas phase react with deuterium adatoms prior to being thermally accommodated to the surface.