A THz-frequency quantum-cascade laser absorption sensing method was developed for quantitative, time-resolved pure rotational spectroscopy of the hydroxyl radical in combustion environments. A systematic wavelength selection process involving consideration of line strength, temperature sensitivity, and spectral interference resulted in the down-selection of a group of rotational OH transitions near 531 cm−1 (15.9 THz). Spectrally-resolved measurements of multiple transitions were achieved using a pulsed quantum-cascade laser (QCL) at a measurement rate of 25 kHz with an integration time of approximately 5 µs. A neighboring water line within the scanning range of the laser enables the water spectra to be measured and subtracted, removing the already minimal water interference near the target OH feature. The THz-range sensor was integrated on a high-enthalpy shock tube to validate the line strengths of the selected OH transitions at or near equilibrium in shock-initiated oxidation of argon-diluted ethylene and oxygen mixtures over a range at temperatures between 1500–3500 K. Quantitative species time histories were also measured during chemical non-equilibrium of ethylene and methanol oxidation to demonstrate the capability to resolve transient formation and destruction of OH during combustion. A detection limit of approximately 0.5 ppm-meter was demonstrated. To the authors’ knowledge, this work represents the first laser absorption sensing of the pure rotational spectra of OH at combustion conditions, exhibiting high potential for numerous applications.