We report a combined photoelectron spectroscopy (PES) and density functional theory (DFT) study on a series of tritantalum oxide clusters, Ta3On-. Well-resolved PES spectra are obtained for Ta3On- (n = 1-8) at several detachment photon energies, yielding electronic structure information which is used for comparison with the DFT calculations. A trend of sequential oxidation is observed as a function of O content until Ta3O8-, which is a stoichiometric cluster. Extensive DFT calculations are performed in search of the lowest energy structures for both the anions and neutrals. The first three O atoms are shown to successively occupy the bridging sites in the Ta-3 triangle, The next three O atoms each occupy a terminal site, with the seventh and eighth O atoms forming a double-bridge and a double-terminal, respectively. The Ta3O7- anion is found to possess a localized electron pair on a single Ta center, making it an interesting molecular model for Ta3+ surface sites. Molecular orbital analyses are performed to elucidate the chemical bonding in the Ta3On- clusters.