If I go to purge the little bit of oxygen in the keg, and the keg is so full that beer comes out of the relief valve, is it safe to assume that the oxygen is purged?
I guess. I try not to fill my kegs past the gas diptube. That way there is less chance of gas getting into my gas lines or even worse, my reg. By some carb tabs and bottle whatever you have left next time.
you still wanna burp it a few times. the CO2 and normal air do mix in the headspace somewhat, although much of the CO2 will fall to the bottom right away.
so you always burp/vent it a few times just to really drive down that 'normal air' ratio.