The air (oxygen, nitrogen, co2, etc) in the keg before filling should be at local atmospheric pressure. When you close the lid and charge the keg to 10 PSI that is at gauge pressure. Gauge pressure disregards the local atmospheric pressure. That is you will be 10 PSI above atmospheric pressure.co2 is heavier then o2. I put a out connector on the co2 line and connect to the keg. I leave the top unlatched and fill for about 10-15 seconds and then close the latch. This should purge any o2 out the top. Then when filling the keg it should displace any left over o2.
When purging, I think the idea is that oxygen is lighter than co2 so it should rise to the top of the keg. Then when you pull the relief valve the oxygen escapes. IMHO, for that to work you should pause bit between pulls of the release valve to allow the gas to reorganize.
Just my process.