When one gets really geeky about minimizing O2, one can fill a keg with sanitizer solution so that all the O2 is displaced. Then push the sanitizer out with CO2, so that you have a keg full of CO2. Then one can rack into the keg via the liquid out post. Also make sure the racking cane and tubing is full of beer, not air when you start, and that can be done by hooking up to a second empty keg until beer is flowing.
Tedious, but effective.