Some people say that you can steep flaked (aka instant) oatmeal like you would steep any other specialty grain. What they say is this will not add any fermentables to your wort, but will add to the mouthfeel/texture of your beer.
Im on the other side with others and say that oatmeal needs to be mashed to get anything besides wet, soggy oats. Its very easy tho. Instead of steeping your oats at 170 for 20-30 minutes, "mash" them at 154 for 60minutes, then sparge them by pouring 170deg water over them. then you continue on like normal. I should also add that they need to be mashed at 154 with another base malt like 2-row or pilsner, pound for pound. so 1lb oats mash with 1lb base malt.
Its very easy and will 100% guaranteed, no argument, impart oatmeal flavor and feel into your beer. 1lb/5gal batch is the high end ive seen (but i could be wrong on that)
there is also a sticky about partial mash brewing that is very informative.