The Os^VIII oxo/hydroxido complexes that are abundant in mild to relatively concentrated basic aqueous solutions are Os^VIIIO₄, [Os^VIIIO₄(OH)]⁻ and two cis-[Os^VIIIO₄(OH)₂]²⁻ species. Os^VIII complexes that contain water ligands are thermodynamically unfavoured w.r.t. the abovementioned species. Os^VIIIO₄ reacts with hydroxide in two, consecutive, elementary coordination sphere expansion steps to form the [Os^VIIIO₄(OH)]⁻ complex and then the cis-[Os^VIIIO₄(OH)₂]²⁻ species. The Gibbs energy of activation for both reactions, in the forward and reverse direction, are in the range of 6–12 kcal mol⁻¹ and are relatively close to diffusion-controlled. The thermodynamic driving force of the first reaction is the bonding energy of the Os^VIII–OH metal-hydroxido ligand, while of the second reaction it is the relatively large hydration energy of the doubly-charged cis-[Os^VIIIO₄(OH)₂]²⁻ product compared to the singly-charged reactants. The DFT-calculated (PBE-D3 functional) in the simulated aqueous phase (COSMO) is −2.4 kcal mol⁻¹ for the first reaction and −0.6 kcal mol⁻¹ for the second reaction and agree to within 1 kcal mol⁻¹ with reported experimental values, at −2.7 and 0.3 kcal mol⁻¹ respectively. From QTAIM and EDA analyses it is deduced that the Os^VIIIO bonding interactions are ionic (closed-shell) and that Os^VIII–OH bonding interactions are polar covalent (dative). In contrast to QTAIM, NCI analysis allowed for the identification of relatively weak intramolecular hydrogen bonding interactions between neighbouring oxo and hydroxido ligands in both [Os^VIIIO₄(OH)]⁻ and cis-[Os^VIIIO₄(OH)₂]²⁻ complexes.