The human body does not grade on a curve. There is as much protein in oatmeal per calorie as there is in red bell peppers and obviously people don't cite red bell peppers as a high protein food, this is true even if for some reason they really prefer to eat red bell peppers.

If you want something from non-animal sources, go for mushrooms and soybeans, which have twice as much protein per calorie as oats. Mushrooms are an under-rated source of protein, as is cottage cheese.