Magnetic circular dichroism (MCD) and absorption spectra are reported over the temperature range 1.7-7 K and the magnetic field range 0-4 T for the hydroxyl free radical isolated in an Ar matrix (OH/Ar). The data indicate a reduction of the orbital angular momentum of the lowest electronic level of the guest. This is interpreted within the framework of a crystal field model involving the Ar atoms of the host lattice. The conclusions are that the OH guest radicals are randomly oriented, occupy at least five distinct sites, and have orbital reduction factors in the range kappa = 1.00 to 0.54.