Green’s Dictionary of Slang

1. (UK Und.) in a team of confidence tricksters, the one who keeps a watch.

1591		Greene Notable Discovery of Coosnage in Grosart (1881–3) X 37: The Art of Cony-catching. These are the eight lawes of villanie leading the high waie to infamie. In High Lawe. The Theefe is called a High lawier. He that setteth the Watch, a Scrippet. He that standeth to watch, an Oake He that is robd, the Martin When he yeeldeth, stouping.
1608		Dekker Belman of London G4: He that stands Centinell and does watch, is an Oke.

2. a rich man, a man of substance.

1608		Dekker Lanthorne and Candle-Light Ch. 4: A Scrivener, who enquiring whether they bee good men and true [...] findes that foure or five, are winde-shaken, and ready to fall into the Lordes handes. Marry, the fift man is an Oake, and theres hope that he cannot bee hewed downe in haste. Upon him therefore the Cittizen buildes so much as comes to five hundred poundes.
c.1698		B.E. Dict. Canting Crew n.p.: an Oak, c. a rich Man, of good Substance and Credit.
1725		New Canting Dict. [as cit. c.1698].
1737, 1759, 1760, 1776		Bailey Universal Etym. Eng. Dict. [as cit. c.1698].
c.1750–1882		B.M. Carew Life and Adventures.
1785, 1788, 1796		Grose Classical Dict. of the Vulgar Tongue.
a.1790		H.T. Potter New Dict. Cant (1795).
1809		G. Andrewes Dict. Sl. and Cant.
1811		Lex. Balatronicum.
1823		Egan Grose’s Classical Dict. of the Vulgar Tongue.
1835		G. Kent Modern Flash Dict.
1848		Flash Dict. in Sinks of London Laid Open.
1862		E. de la Bédollière Londres et les Anglais 316/2: oak, [...] un homme riche et considéré.