I use 1oz but that's in a 3 gallon keg (I ferment in 5g and jump to 3g), but that's a good starting place. On the subject of O2, I water/starsan purge the keg, then hook up the gas to 2 psi, crack the lid and drop the bag in real quick, pressurize to 30 psi to seat the lid, then purge the keg...