Treatment of a dicopper(I,I) complex with nitric oxide produces a dicopper mu-oxo, mu-nitrosyl complex [LCu2(mu-O)(mu-NO)](2+), representing the first structurally characterized mu-oxo, mu-nitrosyl metal complex. This compound can also be synthesized from the reaction of nitrite with an [(LCuCuI)-Cu-II](3+) synthon. Full characterization of the thermal-sensitive [LCu2(mu-O)(mu-NO)](2+) complex with IR, EPR, and X-ray crystallography suggests a localized mixed-valent Cu-III, Cu-II, O2-, NO- formulation. The [Cu-2(mu-O)(mu-NO)](2+) core efficiently oxidizes exogenous substrates, such as phosphine, cyclohexadienes, and isochroman to afford phosphine oxide, benzene, and 1-isochromanone. Since both nitrite and nitric oxide are proposed oxidants in denitrifying methane oxidation, the oxidative reactivity of [Cu-2(mu-O)(mu-NO)](2+) core is potentially relevant to anaerobic methane oxidation observed in methanotrophic archaea.