Green’s Dictionary of Slang

(drugs) an ounce (28g) of a drug.

1933		D. Maurer ‘Junker Lingo’ in AS VIII:2 27: The rations—usually sold at a standard price of $1.00—are designated, according to the type of drug involved, as checks, bindles, or O.Z.’s.
1952		Kerouac letter 27 May in Charters I (1995) 359: An American hipster friend from L.A. laid 5 ozees on me free.
1958		J.C. Holmes Horn 224: Two o.z.’s of ripe Pachuco pot.
1969		R.R. Lingeman Drugs from A to Z (1970) 200: O.Z. An ounce.
1977		B. Davidson Collura (1978) 173: They got an oh-zee for you to taste.
1998		Jayo Felony ‘Ghetto Horror Show’ 🎵 Keys, O-Z’s and quarters.
2000		G. Pelecanos Shame the Devil 105: Tell you what, I got an OZ of cola in the back room.
2007		D. McDonald Luck in the Greater West (2008) 7: An oz [i.e. of amphetamine]? That’d wipe me out, mate.
2001		D. Looser Boobslang [U. Canterbury D.Phil. thesis] 132/2: OZ n. /ouzi/ an ounce of marijuana.