Water adsorption and dissociation on clean and O-covered Co(100), Co(110) and Co(111) surfaces are studied using the density functional theory calculations. The results indicate that molecular water weakly binds to the surfaces and is feasible to desorption from the clean surfaces. Moreover, the pre-adsorption of O atom increases the binding of water to the surfaces, and prominently decreases the activation barriers of water dissociation into OH, especially on Co(110) surface. In contrast, the activation barrier for OH dissociation is slightly affected in the presence of O atom. Overall, this study reveals that O-assisted H2O favorably adsorbs dissociatively, forming OH chemisorbed on the surfaces, which further hinders H2O dissociation, and also illustrates the fact that molecular water dissociation is structure-sensitive on metal surfaces. (C) 2016 Elsevier B.V. All rights reserved.