Working of RC Phase shift Oscillator?
There is stable oscillation if the Barkhausen criterion for oscillation is satisfied.

I.e:


loop gain is 1
(total) loop phase shift is 0°, (or 360° or an integer multiple of 360°):
in this circuit there is 180° phase shift by the inverting input of the OpAmp and another 180° by the phase shift network which results in 360° for exact one frequency.