Hmm...
While it's true that water has no carbs, the water changes the volume of the oatmeal so grams of carbs per cup is different for dry and cooked.
I've seen nutritional listings both ways. It is unfortunate if they don't specify because people will get the wrong answer.
This
USDA site has a handy search feature. Type in "oatmeal" and get 91 different answers, cooked, dry, different brands, with and without milk, honey, etc.