Adsorption and decomposition of water (H2O) over pristine and oxygen (O) assisted Au-Pt(100) surfaces were systematically explored using ab initio calculations based on density functional theory (DFT). To consider the long-range interaction, semi-empirical dispersion correction (D2) was included in all calculations. The most preferable adsorption sites for H2O and its fragments such as OH, O, and H, over clean and O-assisted Au-Pt(100) surfaces were determined by examining adsorption energy with different configurations. Our calculations showed that the H2O prefers top site over Au atom while OH, O, and H prefer to be adsorbed over bridge position. In present study, we determine the best possible co-adsorption sets for considered adsorbates. We further investigated the transition states, dehydrogenation process, and activation energies for extracting H from adsorbed H2O over both pristine and O-aided Au-Pt(100) surfaces. It was found that the O promotes H2O dissociation significantly by diminishing the barrier energy. The decisive role played by O in decomposing H2O molecule is revealed in this work. Our study will further assist experimentalists in designing and synthesizing novel catalysts for dehydrogenation of H2O in hydrogen production. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.