Two neutral and two cationic half-sandwich Os(II)-arene metallodendrimers, of general formula [DAB-PPI-{(η 6 -p-cym)Os((C 7 H 5 NO)-κ 2 -N,O)Cl} n ] and [DAB-PPI-{(η 6 -p-cym)Os((C 6 H 5 N 2 )-κ 2 -N,N)Cl} n ][PF 6 ] n (where n=4 (G1) or 8 (G2)) have been synthesised. In addition, two cationic half-sandwich Os(II)-arene metallodendrimers, containing PTA (1,3,5-triaza-7-phosphatricyclo[3.3.1.1.] decane), of general formula [DAB-PPI-{(η 6 -p-cym)Os((C 7 H 5 NO)-κ 2 -N,O)PTA} n ][PF 6 ] n are also reported. All complexes have been characterised using analytical (i.e. HR-ESI or EI mass spectrometry and elemental analysis) and spectroscopic (i.e. 1 H, 13 C{ 1 H} NMR and infrared) methods. Model mononuclear analogues have been prepared and their molecular structures determined by single-crystal X-ray crystallography. The cytotoxicity of the complexes were evaluated against the A2780 and A2780cisR human ovarian carcinoma cell lines and some display moderate activity.