The nucleation and growth behavior of lithium on a CaO/Mo(001) thin film has been investigated by means of scanning tunneling microscopy and spectroscopy. The Li follows two different growth regimes on the surface. Whereas extended 2D islands develop on top of the defect-free CaO terraces, small 3D deposits decorate a network of domain boundaries that is present in the oxide film. The 2D islands have metallic character, as deduced from a standing wave pattern observed on their surface at low-bias. In contrast, a cationic nature is proposed for the defect-bound 30 species as a result of an electron-transfer from the Li 2s valence orbital into trap states localized in the CaO line defects. Tunneling spectroscopy reveals an unoccupied gap state below the CaO conduction band that originates from Li-O hybridization across the metal-oxide interface. With increasing diameter of the Li islands, this state shifts toward the Fermi level, reflecting the decreasing workfunction at higher Li coverage.