Feedback and phase shift on op-amps
This isn't exact.
You must have the loop phase shift of 180° (and loop gain=1). This is Barkhausen criterion. You can understand this if you draw a block diagram like this:


The equation is:

$$
\frac{V_{out}}{V_{in}}=\frac{A}{1+\beta A}
$$

and tell you (intuitively) that if you want the loop will added to the input then the loop signal must be "negated", and then if I subtract \$-1\$ at something is like if I add \$+1\$.

You could understand better if you see at phase shift oscillator.


You have a inverting amplifier and a passive loop that do 180° phase shift.
Roughly the noise is the signal that start to go through the circuit. If you put this signal with 180° phase shift at inverting input of the amplifier is like you add the signal. Then, oscillation.

Deep study and better math could help more. :-)