Green’s Dictionary of Slang

1. (drugs) opium.

1957		E. & S. Deak Grand Dictionnaire d’Americanismes.
1960, 1975		Wentworth & Flexner DAS.
2001		ONDCP Street Terms 3: Big O — Opium.

2. an orgasm.

1967		Oz 7 23/3: The Big ‘O’ – the only measure of sexual success American men (and women) will accept.
1968		G. Vidal Myra Breckinridge 200: I was about to reach the big O, shrieking with pleasure.
1972		R.A. Wilson Playboy’s Book of Forbidden Words.
2003		T. Dorsey Stingray Shuffle 293: Either of those two things, and it’s no Big O for Maria.

3. (US campus) a person with no personality.

1987

Eble Campus Sl. Nov.

4. (N.Z. prison) an admirable, influential person.

2001

D. Looser Boobslang [U. Canterbury D.Phil. thesis] 17/2: big O, the n. a person who is clever, commendable, brave, ‘the man’ [i.e. ‘the one’].