Dont know if you have done this or not yet, but if you havent I have a suggestion:
Fill the keg with CO2. Put the lid on and attach the gas, after it tank stops pumping, pull the pressure relief valve for a few seconds, then let it fill again, then release again, then let it fill again and remove the gas. Release enough pressure to take the lid off, then remove the lid and you will have an environment that is nearly oxygen free. Oxygen is going to find it's way in there as you fill, but this will certainly limit the amount of oxygen that gets in the beer and thus limit the oxidation.