The design details for a millimeter wave local oscillator for interferometry in the 115-GHz and 230-GHz bands are presented. The system uses a low phase noise 5-GHz synthesizer to provide the phase reference to phase lock a millimeter wave source. Both the 5-GHz synthesizer and the millimeter wave source phase lock circuits use digital phase-frequency detectors. Design tolerances for the local oscillator system are presented, and a coherence test arrangement for evaluating the performance of the system is described. The local oscillator has an interreceiver phase jitter of 7° at 115 GHz and the long-term stability is typically better than 20° pp in 24 hours. The system is currently being used in the Owens Valley three-element millimeter wave interferometer.