The hydroxyl radical (OH) was the first species detected in the interstellar medium by means of its radio (rotational) spectrum, by Weinreb et al. in 1963. It was first observed toward the supernova remnant Cassiopeia A. Many additional detections followed, including the first extragalactic detection in 1971. It was observed in comet Cunningham in 1940 by Swings et al. in the ultraviolet. It was also detected in comets Kohoutek and Halley by Biraud et al. and Stacey et al., respectively. A large part of the Wiki article on OH discusses its astronomical significance (note however that the image of the orbital with the unpaired electron is incorrect).

OH is a very important species. It is very reactive and plays critical roles in both atmospheric and combustion chemistry.