This is the calculator I used for determining my priming sugar.
(you may want to use a couple of calculators and see if they all give the same result)
There is a drop down you can use to get a suggested CO2 volume range for a given style of beer. Unfortunately the range beside Oatmeal stout simply says "?".
I used the value 2.0 which is listed as the low end CO2 volume for a Sweet Stout. My beer temperature is usually 18C when I bottle and I almost always do 23 litre batches. I listed dextrose as my priming ingredient.
Hitting the calculate button gives me 100 grams as my weight of priming sugar.
I'm not sure how important it is to you but I did not take the volume of water (1-2 cups) I used to dissolve the priming sugar into consideration at all.