Family Tree - from This Is Spinal Tap's Christopher Guest - will focus on Tom Chadwick (O'Dowd), a man searching for his long-lost relatives who stumbles across a string of unusual characters and stories.
Bennett will play Tom's best friend, while actress and comedian Nina Conti has also been cast as the lead's sister, Deadline reports.
Amy Seimetz has also joined Family Tree as aspiring writer Ally, who meets Tom when he visits Los Angeles.
The comedy - which will be filmed in a mockumentary-style like much of Guest's past work - started life as a pilot and was ordered to series in August.
Michael McKean (Smallville) and Fred Willard (Anchorman) were previously confirmed to have landed supporting roles.
Family Tree is expected to run for around eight episodes, with BBC Two screening the show in the UK.
