In a total shocker, Conan O'Brien will not be going to FOX ... but instead he's shacking up with TBS.

Conan will host a show Monday - Thursday at 11 PM EST -- getting a 30 minute jump on nemesis Jay Leno.

Once the show debuts, George Lopez's show -- "Lopez Tonight" -- will move to midnight. Short story for George -- the lead-in is a dream come true.

Conan just tweeted -- "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."