An astronomical radio object is classified as a ROCOS if it possesses (a) an optical image with stellar appearance, which is identified with a radio source, and (b) no emission or absorption features in its optical spectrum, except for those due to galactic interstellar medium, with a signal-to-noise ratio at the level of those observable for quasar candidates. About 8% of the known astronomical radio objects satisfy these two criteria and are considered ROCOSes.