The fine-structure transition (2)Pi(1/2)<--(2)Pi(3/2) of the free radical FO has been detected by far-infrared laser magnetic resonance. All the observed transitions are magnetic dipole in character. The spinorbit constant A(0) has been determined experimentally; its value of -196.108 686(50) cm(-1) is consistent with previous estimates. The analysis of a set of 290 transitions leads to the determination of a number of molecular parameters including rotational, centrifugal distortion, spin-orbit, lambda-doubling, magnetic hyperfine, and Zeeman terms. All four magnetic hyperfine structure constants a, b(F), c, d for the F-19 nucleus have been determined and are discussed in terms of the expectation values of the appropriate operators over the electronic wave function.