Effect of external longitudinal magnetic field on the optical Free Induction Decay from a free radical is observed for the first time. The experiments were performed on a rotational transition of hydroxyl radical, OH (23/2(J=1) 23/2(J=0) at 83.8 cm−1) using 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. Longitudinal magnetic field leads to the rotation of the polarization plane of the FID radiation as well as to an additional modulation of the signal intensity. The angle of the rotation of the plane of polarization 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 of selective detection of weak signals of short-lived reactive paramagnetic free radicals from overwhelming signals that originate from stable non-paramagnetic species by polarization discrimination. This work is partially supported by RFBR grant №15-03-05352.