Using density functional theory, the nature of the overlayer formed by dissociation of water on an oxygen covered Pt(111) surface has been studied. This overlayer has been argued to be equivalent to the reaction intermediate in the low-temperature water production reaction on Pt(111). The overlayer is assumed to be formed through either of the two reactions, 2H(2)O(ad)+O-ad-->3OH(ad)+H-ad, or 2H(2)O(ad)+O-ad-->2OH(ad)+H2Oad. Based on experimental data the products of these two reactions are reduced to two adsorbate compositions labeled OH+H2O and 2OH+H. The two compositions are analyzed in terms of their energetics, vibrational spectra, work functions, and simulated scanning tunneling microscopy (STM) images. The OH+H2O composition is found to be 1.4 eV more stable per OH+H2O compared to the 2OH+H composition. Only the vibrational spectrum corresponding to the OH+H2O composition agrees well with experiments, which provides strong additional support for this composition of the overlayer. A large redshift of the water scissoring mode is revealed for this composition. We also find a significant difference between the work functions of the two compositions, which could be used to discriminate between the two compositions. In contrast, we find our simulated STM images of both compositions to be consistent with the observed STM images. (C) 2003 American Institute of Physics.