Appearing on BBC show Something For The Weekend, O'Dowd (The IT Crowd) revealed that he will play Edward in the movie, fiancé to Emily Blunt's princess and enemy to Jack Black's Gulliver.
Tate, meanwhile, will portray the Queen of Lilliput in Fox's adaptation of Jonathan Swift's satirical novel, which centres on a writer who is washed up on an island inhabited by tiny people.
"It's a big, kick-off Hollywood film," Tate is quoted as saying.
O'Dowd can be seen in two movies next month: The Boat That Rocked, to be released on April 1, and Frequently Asked Questions About Time Travel, arriving on April 24.
Gulliver's Travels will open in cinemas in 2010.