Essentially a Stout brewed with a proportion of oatmeal replacing up to 5% of the more usual barley in the brew. This delectable type of beer was popularised in the early 19th Century due to its largely spurious 'health benefits' over other beers, to the point that it used to be prescribed to the elderly as a restorative.
In general, Oatmeal Stouts tend towards the medium to full bodied end of the scale. They have an almost unreal smoothness, richness and a slight sweetness to them, largely due to the higher levels of protein contained due to the use of oats. A personal favourite of mine is Samuel Smiths' Oatmeal Stout, which was one of the first to revive this style of beer in the late 1970s.