While our stout is maturing in fermentation tanks, let's remember where did "oatmeal stout" substyle come from. Multiple sources claim that this modification of stout appeared at the end of 19th century in Great Britain. In particular, Martyn Cornell (Zythophile.wordpress.com) in his book about the history of English beer styles mentions that at least one brewery was producing stout with addition of the oats (unmalted or malted) in 1898. Ron Pattinson adds a number of recipes extracted by him from breweries' archives from the beginning of 20th century. No one knows exactly when this idea came to mind and who was the person but lots of people tend to think that most possible stimulus was the popularity of stout as a healing drink (we remember that they were prescribed to nursing mothers to enforce and stimulate lactation process). Oat was always considered as a healthy grain, so these healing characteristics were much exploited by stout makers and retailers. Moreover, addition of oat gave beer its unique silky and soft texture, making it sweeter than dry irish stouts but not so sweet as milk stouts with lactose addition.
The production of oatmeal stouts lasted until the end of 60th of the last century (until 50th according to other sources). During wartime the usage of oats became a norm (as well as other materials containing starch including potato). However everything comes to the end, so oatmeal stout production closed (according to some sources the latest producer of this beer was the legendary Eldridge Pope from Doncaster, which later became famous for its exceptional barley-wine Thomas Hardy Ale). This style of beer might have the sorrowful fate; however in 1977 Michael Jackson has published his book which transformed the beer world once and for all - World Guide to Beer where the brief history of oatmeal stout was presented. One of American importers decided to reanimate the style and with help of Michael he has contacted the Yorkshire brewery Samuel Smith which brewed this now legendary oatmeal stout in 1990 again. This gave the new impulse to a new history of a style, and we are now the part of this history