You can do it either way. Carb and condition or condition then carb. Either way you want to purge the oxygen out with CO2. Hook up CO2 dispense a little into the keg. Release some air/oxygen until it stops hissing, then dispense a little more to fill the headspace with CO2. On;y purge out of the purge valve or CO2 valve. If you try to purge out of liquid out you will just waste beer.
On Deck: Cream Ale and Blue Moon clone for MIL
Primary: Hopped Up Brown
Bottled: Litehaus Wheat, Orange APA, Rauchbier, Willy's Cream Stout
Kegged: Tank 7 Clone renamed "Le Ferme" (dryhopped cascade, sorachi), Caramel Amber Ale, Trout Bum APA, 3 Dogs APA
Gallons Brewed Since June: 53.5
I seriously cannot imagine a day or life without beer!