Most of the below is from a Brew Wizard column in BYO.
If you wanted to have 10ppm, in 5 gallons (~20L), that would give you 200mg of O2.
There are 32grams of O2 per mole.
So, 200 mg is equal to .00625 moles (.2gm/32 gm of O2 per mole)
The volume of O2 needed can be derived from the Ideal Gas...