The available data on the microwave spectrum of the hydroxyl radical are critically reviewed for information applicable to radio astronomy. Molecular properties such as the rotational constants, spin-orbit, spin-spin and hyperfine coupling constants and centrifugal distortion parameters employed in or derived from the analysis are tabulated. All the observed and predicted transitions of 16OH, 16OD, and 18OH below 300 GHz and lower state energy levels less than 4000 cm−1 are presented for the ground vibrational state. The laboratory data on 17OH is included, but no predicted transitions are presented due to the limited data available. In addition to the transition frequencies the table contains the calculated line strengths and energies of the levels involved in the transition. An extensive bibliography of laboratory and astronomical studies of the hydroxyl radical is presented as an aid to workers in both fields.