Green’s Dictionary of Slang

a beating, a flogging.

1858		A. Mayhew Paved with Gold 141: [I’d] shout at him, and hit him as hard as I could — give him with all my might a good feed of ‘long oats’ and ‘ash beans’.
1935		G. Ingram Cockney Cavalcade 33: Oats, who used to say he was going to give his father’s pony ‘oats’ (whipping – its opposite meaning, as are so many slang expressions), was one of a coster family.