We report the detection of the carbon-chain radical HC5O for the first time in the interstellar medium toward the cold core TMC-1 using the 100 m Green Bank Telescope. We observe four hyperfine components of this radical in the J=17/2\to 15/2 rotational transition that originates from the {}2{{{\Pi }}}1/2 fine structure level of its ground state and calculate an abundance of n/{n}{H2}=1.7× {10}-10, assuming an excitation temperature of {T}{ex}=7 K. No indication of HC3O, HC4O, or HC6O, is found in these or archival observations of the source, while we report tentative evidence for HC7O. We compare calculated upper limits and the abundance of HC5O to predictions based on (1) the abundance trend of the analogous HC n N family in TMC-1 and (2) a gas-grain chemical model. We find that the gas-grain chemical model well reproduces the observed abundance of HC5O, as well as the upper limits of HC3O, HC6O, and HC7O, but HC4O is overproduced. The prospects for astronomical detection of both shorter and longer HC n O chains are discussed.
