Quick Answer
Rolled Oats does not use the same grams-to-cups value as every other ingredient. Using a practical US cup reference, 90 grams of rolled oats is about 1 cup.
That means 100 grams of rolled oats is about 1.11 cups, while 200 grams is about 2.22 cups.
Rolled oats are light and fluffy, with large flakes that leave a lot of air space. One US cup of rolled oats weighs about 90 grams.