Effects of oxygen atom adsorption on work functions of Mo(110) surface and substrate are investigated using first-principles methods based on density functional theory. The calculated results reveal that there exist a most probable site (named f1 site) in the surface oxygen adsorption on Mo(110) surface. Moreover, it is found that work functions of oxygen-adsorbed Mo(110) increase with increasing oxygen coverage, while the adsorption energies of oxygen decrease with increasing oxygen coverage. For a given oxygen coverage such as 0.333ML with surface f1 site, the work functions are insensitive to the distribution of oxygen atoms. In the meantime, the relationship among charge density, surface dipole density and the work function are discussed.