Feedback systems & oscillations
I think that the problem is that you are considering that $$ T = \frac{A}{1+Af} $$ is the actual transfer function of your system (an analog amplifier I guess) and this can be miss leading because that expression is just and approximation for the region of frequencys were the amplifier response is flat. If you want to make assumptions regarding the stability of the system you should work with the full Laplace transform, this is: $$ T(s) = \frac{A(s)}{1+A(s)f(s)} $$
In order to oscillate at a given frequency, the system needs to have conjugate poles on the imaginary-axis at that frequency, but in a practical case, if the system has poles on the right half-plane, as you said, the supply voltage of the amplifiers will move this poles into the j-axis

I don't have enough reputation to comment, but I think that this question that I asked a few weeks ago may be helpful.
https://electronics.stackexchange.com/questions/279365/positive-feedback-with-constant-transfer-functions