I have been doing that for a while. It saves time and I can eat a lot more of it throughtout the day b/c I dont have to use as much water when I mix it in a shake. I do 1 cup of oats with two scoops of whey, and some goodies like glutamine or creatine if its post-workout. Add a table spoon of olive oil and you have a 600 calories shake with 50-60 grams of protein and crabs. Add some water and you are done.
Drink a few of these babies between meals and see what happens.
I dont have any "side effects". If you use old fashioned oats, they are harder to swallow. I use a Giant (supermarket) brand of quick oats that mix easily with water so it doesnt take much to get it down. It doesnt look particularly good but who cares right.