  The double perovskite \bnoo with heptavalent Os ($d^1$) is observed to remain
in the ideal cubic structure ({\it i.e.} without orbital ordering) despite
single occupation of the $t_{2g}$ orbitals, even in the ferromagnetically
ordered phase below 6.8 K. Analysis based on the {\it ab initio} dispersion
expressed in terms of an Os $t_{2g}$-based Wannier function picture, spin-orbit
coupling, Hund's coupling, and strong Coulomb repulsion shows that the magnetic
OsO$_6$ cluster is near a moment-less condition due to spin and orbital
compensation. Quenching (hybridization) then drives the emergence of the small
moment. This compensation, unprecedented in transition metals, arises in a
unified picture that accounts for the observed Mott insulating behavior.
