[en] We present the results of zero-field muon-spin relaxation measurements made on the double perovskite insulators Sr₂ BOsO₆ ($B=\text{Fe},\text{Y},\text{In}$). Spontaneous muon-spin precession indicative of quasistatic long range magnetic ordering is observed in Sr₂FeOsO₆ within the AF1 antiferromagnetic phase for temperatures below $T_{\text{N}}=135\pm <!--\; ??\; -->2$ K. Upon cooling below $T_2\approx <!--\; ???\; -->67$ 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 $T_{\text{N}}=53$ K and 25 K for Sr₂YOsO₆ and Sr₂InOsO₆, respectively. (paper)