We report on experimental and theoretical investigations of the ionization potentials and structures of lithium monoxide clusters. The clusters were produced by a laser vaporization source, laser ionized, and mass selected by a time-of-flight mass spectrometer. Threshold photoionization spectroscopy was performed using photon energies of 3-5.52 eV and 6.4 eV. Ionization potentials of LinO (2 less than or equal to n less than or equal to 70) were derived from the photoionization efficiency curves. The evolution of the ionization potentials as a function of cluster size shows distinct steps at n=10, 22, and 42, and a pronounced odd-even staggering up to n approximate to 42. These steps are in agreement with the shell model for metallic clusters, provided that the oxygen atom localizes two of the lithium valence electrons while leaving the other valence electrons delocalized in a metallic cluster. For the small clusters (n<6), fine structure is observed in the threshold spectra, possibly due to the presence of isomeric states in the cluster beam. Geometries and relative stabilities of the smaller LinO clusters (2 less than or equal to n less than or equal to 8) were computed by means of density functional theory using the B3LYP functional, and adiabatic and vertical ionization potentials were calculated. A comparison with the experimental ionization potentials provides evidence for the greater importance of rigid geometrical structures over metal-like characteristics for the small clusters.