Figure show a RC phase shift oscillator using Op-Amp with gain 5(Vo/Vi).
My assumption : Circuit is generating a sine wave with a peak to peak voltage of 10v.
Consider positive maximum point (+5V) at the output. A fraction of this voltage is feed to inverting terminal of Op-amp with a 180 degree phase shift. ie -1V ( 180 degree phase shifted value 1/5 of 5 ie barkhausen criteria) reaches negative terminal of Op-amp. This -1V is again amplified by the Op-amp and we will get +5v at the output. That means this is a stable condition.
Then why output voltage is oscillating ?