We report the first singlemolecule magnet (SMM) to incorporate the [Os(CN)6]3- moiety. The compound (1) has a trimeric, cyanidebridged MnIII-OsIII-MnIII skeleton in which MnIII designates a [Mn(5Brsalen)(MeOH)]+ unit (5-Brsalen=N,N'-ethylenebis(5-bromosalicylideneiminato)). X-ray crystallographic experiments reveal that 1 is isostructural with the MnIII-FeIII-MnIII analogue (2). Both compounds exhibit a frequency-dependent out-of-phase Χ''(T) alternating current (ac) susceptibility signal that is suggestive of SMM behaviour. From the Arrhenius expression, the effective barrier for 1 is found to be Δeff/kB=19 K (τ(0)=5.0x10-7 s; kB=Boltzmann constant), whereas only the onset (1.5 kHz, 1.8 K) of χ''(T) is observed for 2, thus indicating a higher blocking temperature for 1. The strong spin-orbit coupling present in OsIII isolates the E'1g(1/2)(Oh*) Kramers doublet that exhibits orbital contributions to the single-ion anisotropy. Magnetic susceptibility and inelastic neutron-scattering measurements reveal that substitution of [Fe(CN)6]3- by the [Os(CN)6]3- anion results in larger ferromagnetic, anisotropic exchange interactions going from quasi-Ising exchange interactions in 2 to pure Ising exchange for 1 with J(parallel to)(MnOs)=-30.6 cm-1. The combination of diffuse magnetic orbitals and the Ising-type exchange interaction effectively contributes to a higher blocking temperature. This result is in accordance with theoretical predictions and paves the way for the design of a new generation of SMMs with enhanced SMM properties.