In the Milky Way, the prominent emission of OH at the main line frequencies 1665 and 1667 MHz is associated with star-forming and H II regions. These galactic type I OH masers generally exhibit velocity widths of 1 km/s or less, and the radiation is strongly (in some cases 100) polarized. Studies of various associated constituents observed at optical, IR and radio wavelengths indicate a picture in which the OH maser arises in a dust envelope just outside the Stromgren sphere of gas in a compact H II region. As the ionization front expands through the maser, the OH molecules are dissociated so that the maser eventually disappears. The dynamic lifetime of such masers is only about to years; OH masers therefore may represent a fleeting phenomenon associated with the birth of one or more O-B stars. By studying the main line OH emission seen in nearby galaxies, we may measure the current rate of star formation of such stars in those systems. This determination can be compared with those derived from other indicators, such as thermal radio continuum emission or optical/IR emission from H II regions. Likewise, since the size and luminosity of H II regions is a promising distance indicator, it is critical to understand more fulling the occurrence and physical conditions of star-forming regions in nearby galaxies.
The peak luminosities seen in most extragalactic OH sources are much greater than those of typical galactic type I masers. In the megamasers such as NGC 253, M82 and IC 4553 (Arp 220), peak luminosities exceed Jy . The strongest OH type I maser, W49N, shows a peak luminosity of only about Jy , and typical strong galactic sources are about a factor of ten fainter. Individual maser sources have been detected in the Magellanic Clouds and M33. The difficulty in detecting OH emission from Type I masers in other galaxies arises from the combined requirements of adequate sensitivity (< 1 mJy rms), high resolution ( kHz), broad total bandwidth ( MHz) and both polarizations. Surveys for stellar OH masers would be practical out to 10 Mpc. As these masers occur in the early phases of evolution of massive stars, they might be used to probe, not hampered by extinction, the formation of very massive stars.