A series of six osmium(II) complexes of the type [(η⁶-p-cymene)Os(C^N)X] (X = chlorido or acetato) containing benzimidazole C^N ligands with an ester group as a handle for further functionalization have been synthesized. They exhibit IC₅₀ values in the low micromolar range in a panel of cisplatin (CDDP)-resistant cancer cells (approximately 10× more cytotoxic than CDDP in MCF-7), decrease the levels of intracellular ROS and reduce the NAD⁺ coenzyme, and inhibit tubulin polymerization. This discovery could open the door to a new large family of osmium(II)-based bioconjugates with diverse modes of action.