The paper describes a delay line frequency discriminator technique for frequency locking a 4.7 THz Quantum Cascade Laser (QCL) such as those applied in astronomical heterodyne spectrometers observing the atomic oxygen line. A control signal derived from a 400 MHz subharmonic mixer product and a delay line discriminator stabilizes the laser. This technique significantly reduces complexity and stabilizes the laser linewidth from about 10 MHz in free running mode to 780 KHz when locked for a reduction factor of more than 12 with signal to noise conditions as low as 2 dB. In astronomical terms, this stabilization improvement corresponds to a velocity resolution of the spectrometer of 50 m/s.