Channel 4's popular series sees a group of comedians take on a number of silly challenges presided over by Greg Davies and Alex Horne, with the aim of being crowned Taskmaster.
O'Briain and Millican will also go up against Munya Chawawa, Fern Brady and John Kearns in series 14 of the show.
On Thursday, Taskmaster fans were treated to the second ever Champion of Champions special, as winners from series six to 10 returned to the challenges to get the ultimate title.
— Taskmaster (@taskmaster) June 23, 2022
Richard Herring was named Champion of Champions, and shortly afterwards news of series 14 was announced.
An ad for series 14 saw host Davies sitting on his velvet throne which turned to face a stage, featuring the five new contestants all wearing hula skirts as they greeted him.
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Brady tweeted to her fans: "I guess you guys incessantly tweeting my name to the Taskmaster account paid off."
O'Briain also retweeted the news of his casting, saying: "Oh, and this got announced. It is quite a thing. Can’t wait for you to see it."
Previous winners of the show include Ed Gamble, Kerry Godliman, Liza Tarbuck and Lou Sanders.
Bafta-winning Taskmaster was created by Horne, 43, and first aired on UKTV’s Dave before moving to Channel 4 in 2019.
After the move, the show achieved its best performance on record with its Channel 4 launch, reaching 3.9 million consolidated views.
During the COVID-19 pandemic, Horne hosted #HomeTasking, which saw a series of tasks for the public to complete in their own homes – a selection of which were posted on the show’s YouTube channel and judged by Davies.
A date for the new series of Taskmaster is yet to be confirmed.
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