I think you can add some maltodextrin to the boil, but I can't recall anything that you can add post-fermentation to add body. I may be wrong. I'm sure if there is any way to salvage it, a more experienced brewer will be along shortly to help. There's a lot of very knowledgeable and helpful people here. What was your recipe? Was it only like .5 lbs. of flaked oats?