Yep, you can make oatmeal in the oven. Mine turned out just fine, quite good in fact.
I used a small casserole dish with a lid in a 350F (180C) oven.
I heated 1 cup each of water and milk with a dash of salt in the covered casserole dish for 20 minutes which brought the liquid nearly to a boil. I stirred in 1 cup of old-fashioned oats (not quick-cooking), replaced the lid, and baked without stirring for 20 minutes.
So a 1:2 ratio of oats to nearly boiling liquid.
It is as good as any oatmeal I've ever made. Rice milk should work just as well, I'd go half and half with water the first time you do it, and definitely use a pot or dish (ovenproof, of course) with a tight-fitting lid.
See Is there an easy way to tell if a pan/pot is oven-safe? for advice concerning the cooking vessel. You can be pretty relaxed about any pan that is meant for stovetop use for this application because most modern cookware is made to withstand 350F (180C) easily - even non-stick surfaces or handles.