The program will air Mondays through Thursdays at 11 p.m. Eastern, shifting TBS' "Lopez Tonight," starring George Lopez, to midnight.
O'Brien quickly fired out a celebratory tweet.
"The good news: I will be doing a show on TBS starting in November! The bad news: I'll be playing Rudy on the all new Cosby Show," he posted Monday on Twitter.
The surprise announcement hit the same day O'Brien starts a two-month, nationwide comedy tour in Eugene, Ore., amid persistent reports that he was likely to claim Fox as his new late-night home.
Barring Fox, syndication was widely considered O'Brien's most likely option. Few if anyone mentioned TBS as a plausible destination.
O'Brien began serious talks with TBS just last week, the network said.
"In three months, I've gone from network television to Twitter to performing live in theaters, and now I'm headed to basic cable," O'Brien said in a tongue-in-cheek-toned statement.