The O-protonated CO isomer HOC⁺ was detected in more than a dozen sources by Woods et al. in 1983, a detection that was confirmed twelve years later by Ziurys & Apponi in 1995, with follow-up studies toward other sources in 1997 and 1999. The initial observations were performed with the 14 m telescope at the Five College Radio Astronomy Observatory, which was decommissioned in 2011, and with the 20 m telescope at Onsala Space Observatory. The Ziurys observations were made with the 12 m telescope at the Arizona Radio Observatory at Kitt Peak.

HOC⁺ is considerably less stable than C-protonated CO, HCO⁺, which was detected in 1970. Factors that determine the relative abundances of the two isomers have been the subject of various studies, such as work by Lizst et al. from 2004 and by Smith et al. from 2002.