  We consider cold collisions of OH molecules in the $^2\Pi_{3/2}$ grounds
state, under the influnce of a magnetic field. We find that modest fields of
several thousand gauss can act to suppress inelastic collisions of weak-field
seeking states by two orders of magnitude. We attribute this suppression to two
factors: (i) An indirect coupling of the entrance and the exit channel, in
contrast to the effect of an applied electric field; and (ii) the realtive
motion of the entrance and exit scattering thresholds. In view of these
results, magnetic trapping of OH may prove feasible.
