Andy Richter will serve as sidekick on "Conan."
Richter said he's "thrilled" to be working with O'Brien and he's most exited about "getting out of the house again."
O'Brien added, "This decision was made without my authority. ... I will get to the bottom of this."
Richter and O'Brien first joined forces in 1993 on NBC's "Late Night with Conan O'Brien." Richter was also tapped to appear with O'Brien on "The Tonight Show."
"Conan" will premiere Monday, Nov. 8, at 11 p.m. on TBS.