  A supposedly nonmagnetic 5d$^1$ double perosvkite oxide is investigated by a
combination of spectroscopic and theoretical methods, namely resonant inelastic
X-ray scattering, X-ray absorption spectroscopy, magnetic circular dichroism,
and multiplet ligand field calculations. We found that the large spin-orbit
coupling admixes the 5d $t_{2g}$ and $e_g$ orbitals, covalency raises the 5d
population well above the nominal value, and the local symmetry is lower than
$O_h$. The obtained electronic interactions account for the finite magnetic
moment of Os in this compound and, in general, of 5d$^1$ ions. Our results
provide direct evidence of elusive Jahn-Teller distortions, hinting at a strong
electron-lattice coupling.
