In that case, it depends on how much carbonation you want. An Oatmeal Stout should be between 1.9 and 2.5 volumes, so assuming your beer fermented at 65F, you could go with as little as 2.66 oz of corn sugar and as much as 4.27 oz. The amount of water isn't critical, it just needs to be enough to dissolve the sugar and not completely boil off while sterilizing it.