The gasses will mix, but once you purge a couple of times the o2 is negligible.
In a thread from past years, which I shall not look for, someone posted an analysis of the theoretical mix of o2/co2 when purging. The o2 percentage drops precipitously as you fill and purge with co2. It's never 100% gone, but effectively so.
No doubt that filling with star san, pumping out, and then filling with beer in a closed loop is the best way to get all of the 02 out. It's just not really necessary for most of us.