Despite the fact that a system characterized by a 5d1 spin-orbit-coupled electronic configuration should be nonmagnetic and metallic, Ba2NaOsO6 is a Mott insulator and develops a canted antiferromagnetic long-range order. Abinitio quantum chemical calculations show that, even for perfectly cubic environment, the strong Os 5d-O 2p hybridization (covalency) generates a finite magnetic moment; but mostly, Jahn-Teller effects reduce the local site symmetry and enhances the magnetic moment. Here we probe Ba2Na1−xCaxOsO6 by means of resonant inelastic X-ray scattering (RIXS) and X-ray magnetic circular dichroism (XMCD) in order to access its electronic structure directly and monitor d - d and charge-transfer excitations as fingerprints of the effective ligand field symmetry and strength. We extract quantitative information on spin-orbit coupling, crystal field (including the degree of Jahn-Teller distortions), and covalency characterizing Ba2NaOsO6 and its Ca doped variants, and provide insights into the origin of their unexpected transport and magnetic properties.