Relative partial photoionization cross sections and photoelectron branching ratios have been obtained for the valence bands of osmium tetroxide in the ionization energy range 12 to 18 eV. The photon energies used ranged between 24 and 100 eV. The ionization cross sections of the 2t2 and 2a1 orbitals show evidence of substantial metal character establishing strong Os-O sigma-bonding. Very little evidence is found for pi-bonding.