  Cold, neutral hydroxyl radicals are Stark decelerated and confined within a
magnetic trap consisting of two permanent ring magnets. The OH molecules are
trapped in the ro-vibrational ground state at a density of $\sim10^{6}$
cm$^{-3}$ and temperature of 70 mK. Collisions between the trapped OH sample
and supersonic beams of atomic He and molecular D$_{2}$ are observed and
absolute collision cross sections measured. The He--OH and D$_{2}$--OH
center-of-mass collision energies are tuned from 60 cm$^{-1}$ to 230 cm$^{-1}$
and 145 cm$^{-1}$ to 510 cm$^{-1}$, respectively, yielding evidence of reduced
He--OH inelastic cross sections at energies below 84 cm$^{-1}$, the OH ground
rotational level spacing.
