With oatmeal it gets really sneaky. In the 1890s Maclay of Alloa, Scotland produced a beer called Oat Malt Stout. It was made with 30% oat malt, and the use of oat malt was patented, and the use of the term trademarked.
London brewers wanted to cash in on its popularity.
They started to include flaked oats (no trademark on "oatmeal") in their porter grists -- all of them -- but at only less than 0.5% of the grist usually: just enough that "oatmeal" could legally be put on the label.
Then they took the *exact same beer* and sold some of it as "porter," and some of it as "oatmeal stout." And of course customers would swear they could taste a difference, especially after paying more for "oatmeal stout."