how can i calculate the transfer function of this oscillator?
The first realization is the condition for sine oscillation with an overall phase shift of 4 stages = 360 deg = 180 + 60+60+60 with unity gain. (See Barkhausen criterion)

The problem with your design is the phase shift is 0+3X=360 X=120 is not possible with a 1st order filter, so it oscillate at the unity GBW product of the Op Amps and the Caps will be AC short circuits and not influence the results, as expected.

Point A will have the near saturation level output.

Then each stage is -6dB @ -180-60=-240 deg= +60  


3 stages c 60 deg = 180 with an inverter (with 8dB gain) = 360 = 0 deg 


So it must be corrected as follows with R2/R1=8.

I'll let you compute the frequency where R/Xc impedance ratio gives 60 deg. 







simulate this circuit &ndash; Schematic created using CircuitLab

Knowing a HPF has 45 deg phase shift at the breakpoint at -3dB you have to compute the impedance ratio where the phase shift is 60 deg a common triangle. Then you can prove what I assert is -6dB @ 60 deg so the gain for the inverter needs to be 18dB which is ~ 8:1 for 1+R2/R1.   

I'll let you think about the simple trigonometry.  

In reality, the Op Amp gain reduces when it saturates so slightly > 1 loop gain is tolerated with some slight distortion.. Second, in order to initiate a rapid start, the Cap grows slowly and has no full voltage initial condition, so the LPF phase  shift oscillator is better than the HPF phase shift oscillator as this one represents.  Also the Op Amps need a split supply or else the R's must go to V+/2.