A maser source in which the hydroxyl (OH) molecule is excited to maser action. There are four OH spectral lines at frequencies of 1612, 1665, 1667, and 1720 MHz (a wavelength of about 18 cm). Other OH spectral lines at higher frequencies are also sometimes excited as masers. OH masers are found in star-forming regions, circumstellar envelopes, supernova remnants, and comets, and in active galactic nuclei in the form of megamasers.