Oatmeal is one of those utilitarian grains which you can buy relatively cheaply back home. In fact, besides the health benefits, one of the main reasons for choosing plain oatmeal as your breakfast grain of choice is how economical it is. A small box of (bulk, plain) oatmeal (300 grams/10.6 oz.) is nearly $4.00 (U.S.) in Japan. That's over $5 a pound and you can buy the same thing for under a dollar a pound in most parts of America. I regularly use oatmeal for baking, and these small boxes are something I can burn through pretty quickly.
I won't miss the absurdly high price of oatmeal.