I'm not worried at all. as i said, I've never had a problem. I suspect, as Mark (SCerv) would point out, unless we are filtering our beer there is plenty of yeast still active to clean up any introduced o2. I'm just curious.
I'm not talking about diffusion, the idea is that the o2 molecules are forced out when you open the PRV but why would that happen? the o2 pressue inside the keg is at atmopheric. there wouldn't be any exchange of o2 because there wouldn't be a pressure differential inside and outside. when you open the PRV the partial pressure of co2 inside and outside the keg rapidly equalizes. But with the o2, because we have not added any additional o2, there is no equalization so no o2 leaves.
It should be testable actually. put an o2 sensor in a keg, off the bat it should read = atmospheric concentrations. Pressurize the keg with co2. there are now the same number of o2 molecules as there was before but lots more co2. when you pull the PRV until it stops hissing the o2 sensor will either read the same as at the begining or much lower. I don't know that it matters but it's interesting.