The electronic structure of Ni2Ti4O is studied using first-principles calculations based on the linear-muffin-tin-orbital method. The role of oxygen in stabilizing the metallic NiTi2-type structure is analysed by site-projected energy calculations which indicate that the Ti(f) sites with nonicosahedral symmetry determine the structural stability of Ni2Ti4O. The origin of this behaviour, contrary to that found for NiTi2, is due to the effects of 2p(O)-3d(Ti(f)) hybridization.