A series of cationic metalla-rectangles of the general formula [(p-cymene)4Os4(OO.intrsec.OO)2(N.intrsec.N)2]4+ have been obtained in methanol from the dinuclear arene osmium precursors [(p-cymene)2Os2(OO.intrsec.OO)2Cl2] (OO.intrsec.OO = 2,5-dihydroxy-1,4-benzoquinonato (dhbq), 2,5-dichloro-1,4-benzoquinonato (dcbq)) by reaction with bipyridine linkers (N.intrsec.N = 4,4'-bipyridine, 1,2-bis(4-pyridyl)ethylene) in the presence of AgCF3SO3. All complexes were isolated as triflate salts and characterized by NMR, IR and UV-visible spectroscopy. The cytotoxicities of the dinuclear and tetranuclear osmium complexes were established using ovarian A2780 cancer cell lines. The most active metalla-rectangle, [(p-cymene)4Os4(dhbq)2(4,4'-bipyridine)2]4+, shows an IC50 value of 5.7 μM (comparable to cisplatin) against A2780 cancer cells and 7.5 μM against the cisplatin resistant A2780cisR cells.