  We report the crystal structure, magnetization and neutron scattering
measurements on the double perovskite Ba$_2$YOsO$_6$. The $Fm\overline{3}m$
space group is found both at 290~K and 3.5~K with cell constants $a_0 =
8.3541(4)$~{\AA} and $8.3435(4)$~\AA, respectively. Os$^{5+}$ ($5d^3$) ions
occupy a non-distorted, geometrically frustrated face-centered-cubic (FCC)
lattice. A Curie-Weiss temperature $\theta = -772$~K suggests the presence of a
large antiferromagnetic interaction and a high degree of magnetic frustration.
A magnetic transition to long range antiferromagnetic order, consistent with a
Type I FCC state below $T_{\rm N} \sim 69$~K, is revealed by magnetization,
Fisher heat capacity and elastic neutron scattering, with an ordered moment of
1.65(6)~$\mu_B$ on Os$^{5+}$. The ordered moment is much reduced from either
the expected spin only value of $\sim 3 \mu_B$ or the value appropriate to
$4d^3$ Ru$^{5+}$ in isostructural Ba$_2$YRuO$_6$ of 2.2(1)~$\mu_B$, suggesting
a role for spin orbit coupling (SOC). Triple axis neutron scattering
measurements of the order parameter suggest an additional first-order
transition at $T = 67.45$~K, and the existence of a second ordered state. %
Time-of-flight inelastic neutron results reveal a large spin gap $\Delta \sim
17$~meV, unexpected for an orbitally quenched, $d^3$ electronic configuration.
We discuss this in the context of the $\sim 5$~meV spin gap observed in the
related Ru$^{5+}$, $4d^3$ cubic double perovskite Ba$_2$YRuO$_6$, and attribute
the $\sim 3$ times larger gap to stronger SOC present in this heavier, $5d$,
osmate system.
