You can fill the keg with sanitizer, the push it out with CO2 and y ou have a keg with no O2. You can fill with CO2 and bleed the keg, then repeat the cycle, as you have successively less O2 each time.
When you open the lid some O2 will defuse in as you fill, but the difussion is not instantaneous. Do the repeated purge on the head space once filled with beer and the lid is back on.
If you really want the beer to remain fresh, fill through the dip tube, pushing the beer out of the fermenter with CO2 - a closed transfer. For safety only use 2 PSI, that is all it takes. If you want to speed the process, place the fermenter higher up, start the transfer, and gravity gives a small assistance.