The adsorption of atomic oxygen on the Nb (110) surface is systematically investigated through the first-principles method for oxygen coverage ranging from 0.25 to 1 monolayer (ML)．It is found that the hollow site is the most energetically favorable for the whole coverage range considered and that the long-bridge site takes the second place．The work function increases almost linearly with the increase of oxygen coverage for the long-bridge site adsorption, whereas for the hollow site adsorption the work function decreases when the coverage is 0.25 ML or 0.5 ML and increases when the coverage is 0.75 ML or 1 ML．Using the planar averaged charge density and the dipole moment change we can explain the complicated change of work function induced by atomic oxygen adsorption．In addition, the interaction between O and Nb is analyzed by the surface atomic structure and electronic density of states．