Green’s Dictionary of Slang

1. (US campus) having a positive time.

1985

Eble Campus Sl. Oct. 1: be on one – things are going well. ‘You’re on one today.’.

2. behaving crazily.

1997–2001		Online Sl. Dict. 🌐 on one v 1. to act out of the ordinary, as if one were on medication. (“My mom was on one yesterday so I couldn’t make any phone calls.”).
2013		K. Sampson Killing Pool 58: I can tail this on-one knob without too much trouble.
2020		A. Parks Bobby March Will Live Forever 75: ‘[Y]ou know what Cooper’s like when he’s on one. He won’t listen’.