Green’s Dictionary of Slang

1. an atom, the tiniest amount.

1873

Sl. Dict. 240: Oat an atom. Probably corruption of iota, or perhaps from the small size of an oat. ‘I never got an OAT of it,’ I never received the smallest portion.

2. a penny, a halfpenny, the smallest amount of money; thus in pl., money.

1900		‘Doss Chiderdoss’ ‘An Exacting Swain’ Sporting Times 24 Feb. 1/4: I’ll stake ev’ry oat in my ‘sky’ / To a pair of kid gloves or a new sailor hat.
1905		Sporting Times 1 Apr. 3/2: If we don’t come down with the dibs on our own, the County Council, or one of those delightful institutions in our midst for wringing the last oat out of a long-suffering public, will be down upon us with a fine Jubilee rate.
1920		‘Sax Rohmer’ Golden Scorpion 231: An’ not a ‘oat’ in me pocket— not a ‘bean’! Broke to the wide.
1928		V. Palmer Passage 42: Uncle Tony’s been rubbing it into me that I don’t earn my oats [...] Says he was making money by the time he was twelve.