  MERLIN measurements of 1.6-GHz OH masers associated with Orion-BN/KL are
presented, and the data are compared with data on other masers, molecular
lines, compact radio continuum sources and infrared sources in the region. OH
masers are detected over an area 30 arcsec in diameter, with the majority lying
along an approximately E-W structure that extends for approx. 18~arcsec,
encompassing the infrared sources IRc2, IRc6 and IRc7. Radial velocities range
from -13 to +42 km/s. The system of OH masers shows a velocity gradient
together with non-circular motions. The kinematics are modelled in terms of an
expanding and rotating disc or torus. The rotation axis is found to be in the
same direction as the molecular outflow. There is an inner cavity of radius
approx 1300 au with no OH masers. The inner cavity, like the water `shell'
masers and SiO masers, is centred on radio source I. Some of the OH masers
occur in velocity-coherent strings or arcs that are longer than 5~arcsec (2250
au). One such feature, Stream A, is a linear structure at position angle approx
45 degree, lying between IRc2 and BN. We suggest that these masers trace shock
fronts, and have appeared, like a vapour trail, 200 yr after the passage of the
runaway star BN. The radio proper motions of BN, source I and source n project
back to a region near the base of Stream A that is largely devoid of OH masers.
The 1612-MHz masers are kinematically distinct from the other OH masers. They
are also more widely distributed and appear to be associated with the outflow
as traced by water masers and by the 2.12-micron emission from shocked H2 The
magnetic field traced by the OH masers ranges from 1.8 to 16.3 mG, with a
possible reversal. No OH masers were found associated with even the most
prominent proplyds within 10 arcsec of Theta1 Ori C.
