  We present the results of zero-field muon-spin relaxation measurements made
on the double perovskite insulators Sr$_{2}B$OsO$_6$ ($B={\rm Fe, Y, In}$).
Spontaneous muon-spin precession indicative of quasistatic long range magnetic
ordering is observed in Sr$_{2}$FeOsO$_6$ within the AF1 antiferromagnetic
phase for temperatures below $T_{\rm N}=135 \pm 2~{\rm K}$. Upon cooling below
$T_2 \approx 67~{\rm K}$ the oscillations cease to be resolvable owing to the
coexistence of the AF1 and AF2 phases, which leads to a broader range of
internal magnetic fields. Using density functional calculations we identify a
candidate muon stopping site within the unit cell, which dipole field
simulations show to be consistent with the proposed magnetic structure. The
possibility of incommensurate magnetic ordering is discussed for temperatures
below $T_{\rm N}=53~{\rm K}$ and 25~K for Sr$_{2}$YOsO$_6$ and
Sr$_{2}$InOsO$_6$, respectively.
