  We propose a model for the multi-orbital material Sr$_2$CrOsO$_6$ (SCOO), an
insulator with remarkable magnetic properties and the highest $T_c \simeq 725$
K among {\em all} perovskites with a net moment. We derive a new criterion for
the Mott transition $(\widetilde{U}_{1} \widetilde{U}_{2})^{1/2}>2.5W$ using
slave rotor mean field theory, where $W$ is the bandwidth and
$\widetilde{U}_{1(2)}$ are the effective Coulomb interactions on Cr(Os)
including Hund's coupling. We show that SCOO is a Mott insulator, where the
large Cr $\widetilde{U}_{1}$ compensates for the small Os $\widetilde{U}_{2}$.
The spin sector is described by a frustrated antiferromagnetic Heisenberg model
that naturally explains the net moment arising from canting and also the
observed non-monotonic magnetization $M(T)$. We predict characteristic magnetic
structure factor peaks that can be probed by neutron experiments.
