The microwave spectrum of phosphenous fluoride, OFF, has been measured using a pulsed-jet cavity Fourier transform microwave spectrometer. With the exception of a mass spectroscopic detection of the molecule, chemically prepared for use in a matrix IR study, this is the first observation of free gas-phase OFF. The samples were prepared from mixtures of PF3 and O-2 in Ne carrier gas, using an electric discharge. Rotational transitions of two isotopomers ((OPF)-O-16-P-31-F-19 and (OPF)-O-18-P-31-F-19) have been measured in the 4-26 GHz frequency range. The determined rotational constants have been used to calculate r(0), r(7), and approximate r(e) molecular geometries. In contrast to the nitrogen analogue, ONF, OFF has been found to show no irregularities in its geometry. Small hyperfine splittings due to the spin-1/2 P-31 and F-19 nuclei have been analyzed in terms of nuclear spin-rotation interactions. Because the determined coupling constants were of similar magnitudes and could not be unambiguously assigned, the related nuclear shielding parameters have been derived using both possible assignments. The spin-rotation coupling constants are compared with those calculated using ab initio techniques, and the F-19 nuclear shieldings are compared with those derived for the nitrogen analogue ONF.