Green’s Dictionary of Slang

big O n.

[initial letter/zero]

1. (drugs) opium.

[US]E. & S. Deak Grand Dictionnaire d’Americanismes.
[US]Wentworth & Flexner DAS.
[US]ONDCP Street Terms 3: Big O — Opium.

2. an orgasm.

[UK]Oz 7 23/3: The Big ‘O’ – the only measure of sexual success American men (and women) will accept.
G. Vidal Myra Breckinridge 200: I was about to reach the big O, shrieking with pleasure.
[US]R.A. Wilson Playboy’s Book of Forbidden Words.
[US]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.

[US]Eble Campus Sl. Nov.