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.