  We analyze full-polarization VLBA data of ground-state, main-line OH masers
in 18 massive star-forming regions previously presented in a companion paper.
The OH masers often arise in the shocked neutral gas surrounding ultracompact
Hii regions. Magnetic fields as deduced from OH maser Zeeman splitting are
highly ordered, both on the scale of a source as well as the maser clustering
scale of ~10^15 cm. Results from our large sample show that this clustering
scale appears to be universal to these masers. OH masers around ultracompact
Hii regions live ~10^4 years and then turn off abruptly, rather than weakening
gradually with time. These masers have a wide range of polarization properties.
At one extreme (e.g., W75 N), pi-components are detected and the polarization
position angles of maser spots show some organization. At the other extreme
(e.g., W51 e1/e2), almost no linear polarization is detected and total
polarization fractions can be substantially less than unity. A typical source
has properties intermediate to these two extremes. In contrast to the well
ordered magnetic field inferred from Zeeman splitting, there is generally no
clear pattern in the distribution of polarization position angles. This can be
explained if Faraday rotation in a typical OH maser source is large on a maser
amplification length but small on a single (e-folding) gain length. Increasing
or decreasing Faraday rotation by a factor of ~5 among different sources can
explain the observed variation in polarization properties. We suggest that
almost all pi-components acquire a signficant amount of circular polarization
from low-gain stimulated emission of a sigma-component from OH appropriately
shifted in velocity and lying along the propagation path.
