I asked what you fermented in because the best way to remove all oxygen is to fill the beer with StarSan or water and push out with CO2, then fill with your beer through the 'out'. With this method, you could just have a gallon to rack in there and it wouldn't matter. This method uses the least amount of CO2 but it pretty much requires your fermentation vessel to hold 2psi.
Another option is to fill through the lid, seal, pressurize to 30psi, purge and repeat 5-10 times. This works best if you can add priming sugar to the keg and let it naturally carbonate, which will scavenge the remaining O2, but that takes another 3 weeks. Nobody wants to wait that long on their first keg but it is something to think about for the future.