An X-band low-phase noise planar oscillator employing the substrate integrated waveguide (SIW) active resonator is demonstrated. By compensating the losses in the SIW cavity with an active feedback loop, the Q-factor of the SIW active resonator is greatly improved. The measured results show a loaded Q-factor of 1569 and unloaded Q-factor of 8782, which is very high among other planar resonators. A simplified generalised phase noise condition and its optimisation approach are proposed for the low-phase noise oscillator design. To validate the proposed optimisation approach, experimental prototypes of oscillators using different design parameters and resonators are fabricated. The measured results show that the optimised SIW active resonator oscillator possesses low-phase noise of −109.2 dBc/Hz at 100 kHz at X-band, which is 17 and 9 dB better than the microstrip resonator oscillator and SIW passive resonator oscillator, and is comparable with the dielectric resonator oscillator measured in this study.