Green’s Dictionary of Slang

1. an urban rowdy, usu. in a gang [note ‘No trace of this odd appellation has yet been found except that the author of a ludicrous pamphlet has taken the name of Oliver Oat-meale’ (Nares, Glossary, 1822)].

c.1639

Dekker & Ford Sun’s-darling I 4: Do mad pranck with Roaring boies and oatmeals.

2. corruption, criminality .

1946

T. Thursday ‘Raw, Medium, and Well Done’ in Blue Ribbon Western June 🌐 Boy [...] ain’t that some oatmeal, huh? Why, the crooked crocodile should get a load of buckshot in the pants!