The effect of preadsorbed oxygen on the subsequent adsorption and reactions of water on Ru(0001) has been studied using low temperature scanning tunneling microscopy and DFT calculations. Experiments were carried out for O coverages close to 0.25 ML. It was found that no dissociation of water takes place up to the desorption temperature of similar to 180-230 K. DFT calculations show that intact water on 0(2 x 2)/Ru(0001) is similar to 0.49 eV more stable than the dissociation products, H and OH, at their preferred fee and top adsorption sites.