I personally used to give it a blast anyway, with the lid partly screwed on, just to expel the air in the keg. Then turn the lid fully tight. The yeast doesn't need oxygen to ferment the priming sugar at this stage, only when it is growing.
It is the priming sugar that will make the fizz. (You'd get very little forced-carbonation if the keg has a pressure release valve, by the way, and I think it's only really in metal kegs that you'd do that).