A photochemical synthesis of [CpOs(CH3CN)3]+ is reported (Cp = η5-cyclopentadienyl). Photolysis of [CpOs(Bz)]+ (Bz = η6-benzene) in acetonitrile solution gives [CpOs(CH3CN)3]+, but only 30% conversion is achieved before significant photochemically induced decomposition of [CpOs(CH3CN)3]+ occurs. Photolysis of acetonitrile solutions of [CpOs(Bz)]+ that contain biphenyl eliminates this side reaction and allows the high-yield preparation of [CpOs(CH3CN)3]+. Quantum yields for the photodearylation reaction are identical in the presence or absence of biphenyl, suggesting that biphenyl acts as a passive filter, slowing the photochemical decomposition of [CpOs(CH3CN)3]+, [CpOs(CH3CN)3]+ reacts with arenes, CO, and polypyrazolylborate ligands, providing a high-yield route to CpOsL3 compounds under mild conditions. Compounds that were synthesized and fully characterized include [CpOs(η6-anthracene)]PF6. [CpOs(η6-rubrene)]PF6, CpOs(BPz4) (BPz4- = tetrakis(1-pyrazolyl)borate), CpOs(HBPz3) (HBPz3 = hydrotris(1-pyrazolyl)borate), [CpOs(CH3CN)2CO]PF6, and CpOs(CO)2Br. 1H NMR evidence for an additional species ([(CpOs)(CpOs(CH3CN))(μ-η6, η4-anth)]2+) that contains an η6 and an η4 CpOs+ group is presented. Intriguing differences between the reactivity of [CpOs(CH3CN)3]+ and [CpRu(CH3CN)3]+ with rubrene are observed, with the osmium complex showing a remarkable kinetic preference for the naphthacene backbone relative to the pendant phenyl rings.