  The effect of pressure and temperature on the Raman-active phonon mode of
osmium metal has been investigated for pressures up to 20 GPa and temperatures
in the range 10--300 K. Under hydrostatic conditions (He pressure medium) the
phonon frequency increases at a rate of 0.73(5) cm^{-1}/GPa (T = 300 K). A
large temperature-induced and wavelength-dependent frequency shift of the
phonon frequency is observed, of which only a small fraction can be associated
with the thermal volume change. The main contribution to the temperature
dependence of the phonon frequency is rather attributed to non-adiabatic
effects in the electron-phonon interaction, which explains also the observation
of an increasing phonon line width upon cooling. The phonon line width and the
pressure-induced frequency shift were found to be unusually sensitive to shear
stress.
