Dioxygen addition to the 16-electron complexes [OsX(P-P)(2)](+) (3) gives the dioxygen adducts [OsCl(eta (2)-O-2)(P-P)(2)](+) (3), which in turn react with HCl gas to give the novel osmium(IV) oxo complexes trans-[OsX(O)(P-P)(2)](+) (5) (X = Cl, Br; P-P = 1,2-bis(dicyclohexylphosphino)ethane (dcpe), 1,2;bis(diethylphosphino)ethane (depe), 1,2-bis((2R,5R)-2,5-dimethylphosphopholano) (Me-duphos)). The complexes [OsX(dcpe)(2)](+) (X = Cl, Br) (3) are studied by X-ray crystallography and are shown to have a "Y-shaped" coordination geometry in the equatorial plane. The X-ray structural analysis of [OsCl(eta (2)-P-2)(dcpe)(2)](+) (4a) reveals an exceptionally short O-O bond (1.315(5) Angstrom). trans-[OsCl(O)(dcpe)(2)](+) (5a), the first oxo complex of osmium(IV) investigated crystallographically, exhibits a long Os-O distance of 1.834(3) Angstrom. The reactivity of 4 and 5 as oxidants is described. The dioxygen complex 4a transfers one oxygen atom to PPh3 (to give Ph3PO) or oxidizes iodide ions to triiodide ions in the presence of anhydrous HCl. In both reactions, the corresponding oxo species 5a is quantitatively formed as the only metal-containing product. Oxo complexes 5 are surprisingly stable and unreactive toward standard reducing agents such as phosphines.