We report the first single-molecule magnet (SMM) to incorporate the [Os(CN)₆]³⁻ moiety. The compound (1) has a trimeric, cyanide-bridged Mn^III-Os^III-Mn^III skeleton in which Mn^III designates a [Mn(5-Brsalen)(MeOH)]⁺ unit (5-Brsalen=N,N′-ethylenebis(5-bromosalicylideneiminato)). X-ray crystallographic experiments reveal that 1 is isostructural with the Mn^III-Fe^III-Mn^III 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 Δ_eff/k_B=19 K (τ₀=5.0×10⁻⁷ s; k_B=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 Os^III isolates the E′_1g(1/2)(O_h*) 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 =−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.