The recent synthesis of (pentamethylcyclopentadienyl)osmium(VI) pentahydride (Cp*OsH5) completes the series of high-oxidation-state cyclopentadienyl metal polyhydrides. A single hydride resonance in the proton NMR spectrum indicated that the complex contained either five equivalent hydride ligands or inequivalent hydrides in rapid exchange, Vibrational studies were inconclusive as to whether the molecule adopted a C-5v (pseudo-pentagonal-pyramid with the Cp ring occupying the axial position) or a pseudo-C-4v (pseudo-octahedral with the Cp ring considered as occupying one site) orientation in the ground state. In this study, ab initio calculations with effective core potentials were performed on pentahydrido(cyelopentadienyI)osmium(VI), CpOsH(5), to determine its structure, vibrational spectrum, and rearrangement mechanism. Geometry optimizations at the Hartree-Fock (HF) and second-order Moller-Plesset (MP2) levels of theory and energy calculations at the MP3, configuration interaction singles and doubles (CISD), coupled cluster singles and doubles (CCSD), and coupled cluster singles and doubles with perturbative estimate of triple excitations (CCSD(T)) levels place the pseudo-C-4v, structure lower in energy than the C-5v structure. When compared to the experimental results, the calculated frequencies confirm that the pseudo-C-4v geometry is the experimentally observed conformation. The topography of the MP2 potential energy surface supports the notion of a highly fluxional pseudo-C-4v geometry which exchanges hydride ligands through a Bailor or trigonal-twist mechanism without passing through the C-5v geometry.