Add the oats to the bowl of a food processor fitted with the blade attachment. Blend on high speed until a fine flour forms, usually a couple of minutes.
200 grams rolled oats
Your oat flour is ready to use. If measured in grams, you'll have the correct amount. If using cups, you'll have to measure the oat flour after blending to get about the right amount (but good gluten-free recipes usually provide weight measurements for accuracy).
Notes
Generally speaking, this amount should make about two cups of oat flour.