How is the closed loop transfer function defined for an oscillator?
Just regard it as an amplifier with feedback where the real input is set to a value of zero: -




  in negative feedback the feedback signal is added to the input signal,
  in positive feedback is subtracted. So in the first situation we use
  and adder circuit, in the second one a subtractor circuit.


Feedback +/- input is the same as feedback +/- 0 when the input is zero.