[em3rgent0rdr]

Sounds like you are stating https://en.wikipedia.org/wiki/Barkhausen_stability_criterion (1921):

"if A is the gain of the amplifying element in the circuit and β(jω) is the transfer function of the feedback path, so βA is the loop gain around the feedback loop of the circuit, the circuit will sustain steady-state oscillations only at frequencies for which:

1: The loop gain is equal to unity in absolute magnitude, that is, |βA|=1, and 2: the phase shift around the loop is zero or an integer multiple of 2π: ∠βA=2πn,n∈{0,1,2,…}"

[em3rgent0rdr]

But you said "phase is 180 degrees" which is 1π, while Barkhausen instead says integer multiple of 2π.

[spot5010]

Found an article which talks about the phase margin, referencing 80 degrees (pi). https://en.wikipedia.org/wiki/Phase_margin