The following circuit diagram show us another kind of CMOS oscillator. It’s will always oscillate because its a loop of three inverters. It would oscillate at a very high frequency (MHz) without the RC components depending on the propagation delays of the inverters.
The RC branches give a phase shift of 60′ at some frequency, so the overall phase shift will be 180′ and the circuit will oscillate at this frequency. The oscillation frequency was 918 Hz with R=330k and C=0.001uF. The waveform was approximately sinusoidal and has peak to peak value 6.5V with Vdd=12V. I used a CD4049 for these circuits but HC inverter would work as well. We can learn more about relaxation oscillators in the page devoted to them.