Effect of external longitudinal magnetic field on the optical Free Induction Decay (FID) from a free radical was observed for the first time. The experiments were performed on the rotational transition of hydroxyl radical OH (233/2(J=1) Å 233/2(J=0) at 83.8 cmí1) using the Terahertz Free Electron Laser. In contrast to the results of the experiments with a stable paramagnetic molecule NO, the observed effect of external magnetic field on the Free Induction Decay from hydroxyl radicals is more complicated. A longitudinal magnetic field leads to rotation of the polarization plane of the FID radiation, as well as to additional modulation of the signal intensity. The angle of rotation of the polarization plane is large, in agreement with the theoretical predictions. The observed FID kinetics in the time domain are in semi-quantitative agreement with the modeling. This observation opens an opportunity for selective detection by polarization discrimination of weak signals of short-lived reactive paramagnetic free radicals from overwhelming signals that originate from stable non-paramagnetic species.