Abstract The influence of spin-orbit coupling (SOC) on the physical properties of the 5 d 2 system Sr 2 MgOsO 6 is probed via a combination of magnetometry, specific heat measurements, elastic and inelastic neutron scattering and density functional theory calculations. Although a significant degree of frustration is expected, we find that Sr 2 MgOsO 6 orders in a type I antiferromagnetic structure at the remarkably high temperature of 108 K. The measurements presented allow for the first accurate quantification of the size of the magnetic moment in a 5 d 2 system of 0.60(2) μ B  –a significantly reduced moment from the expected value for such a system. Furthermore, significant anisotropy is identified via a spin excitation gap and we confirm by first principles calculations that SOC not only provides the magnetocrystalline anisotropy, but also plays a crucial role in determining both the ground state magnetic order and the size of the local moment in this compound. Through comparison to Sr 2 ScOsO 6 , it is demonstrated that SOC-induced anisotropy has the ability to relieve frustration in 5 d 2 systems relative to their 5 d 3 counterparts, providing an explanation of the high T N found in Sr 2 MgOsO 6 .