The elusive heterocumulenic radical OCCNO˙ and its isotopologues OC¹³CNO˙ and OCC¹⁵NO˙ have been prepared by reacting photolytically generated unsaturated carbene OCC/OC¹³C with ˙NO/¹⁵˙NO in cryogenic N₂-, Ar-, and Ne-matrices. Upon UV-light (365 nm) irradiation, the C–C bond in OCCNO˙ breaks and yields a long-sought ground-state radical CNO˙ (X²Π), which has also been identified with matrix-isolation infrared spectroscopy.