Multisubstituted isoquinoline and pyridine N-oxides have been prepared by Rh(III)-catalyzed cyclization of oximes and diazo compounds via aryl and vinylic C-H activation. This intermolecular annulation involving tandem C-H activation, cyclization, and condensation steps proceeds under mild conditions, obviates the need for oxidants, releases N-2 and H2O as the byproducts, and displays a broad substituent scope.