John Goodman, Louis C.K and Elle Fanning also join the celebrated cast.
It's a major coup for O'Gorman who auditioned for the role only a month ago by Skype from his home in Auckland.
Two weeks later, he was flown to Los Angeles by director Jay Roach (Austin Powers, Meet the Parents) and screen-tested in person. Then came the fittings for the facial prosthetics, including the famous Douglas chin dimple, and the 1940s wardrobe.
"The movie is a huge boost for Dean's profile. He's extremely honoured to be working alongside such a stellar cast, who all signed on knowing he's one of the lead roles," said his New Zealand agent, Imogen Johnson.
"It's a testament to Dean's reputation. He's humble and hard-working. People love working with him and he's always in demand."
O'Gorman, who lives permanently in New Zealand, returns to the screens as Fili in The Hobbit: The Battle of Five Armies, which will be released in New Zealand on December 17.
The Kiwi's acting repertoire is diverse, from blockbusters and independent films to TV series, including local productions The Almighty Johnsons and Go Girls.
In Trumbo, O'Gorman will play the iconic actor during a controversial period in Hollywood's history in which the screenwriter refused to testify before the House Un-American Activities Committee in 1947 during the investigation of Communist influences in the motion picture industry.
The film is based on Bruce Cook's 1977 book.
Trumbo won two Oscars - under the pseudonyms Robert Rich and Ian McLellan Hunter - while blacklisted, for The Brave One and Roman Holiday.
His screenwriting role on Spartacus, for which he was belatedly credited, is thanks to Douglas, who was one of the film's executive producers. He influenced the decision to hire Trumbo to write the screenplay, and his praise of Trumbo has been cited as the beginning of the end of the blacklist.
Big fella goes silent
Kim Dotcom has gone to ground after declaring defeat on Saturday night. His typical daily Twitter diatribe has gone silent. But what of the email he brandished before the election pronouncing unequivocal proof John Key misled the House over when he was aware of Dotcom's existence?
It was referred to Parliament's privileges committee by Mana leader Hone Harawira, but rejected by the Speaker because Parliament had been dissolved ahead of the election.
Will the email still go to the privileges committee now the Internet Party has no position in power, or friends with it?
That's unlikely. Laila Harre, who faced defeasance as party leader and is presumably now on the job hunt, took to Twitter to explain.
"It will need an MP to take it there," Harre told a follower. However, she couldn't name one MP willing to take up the quest. So, what next? Harre said it was up to "Kim and his lawyer" to decide the next move.
Congrats of the week
• Congratulations to Lorde who was named yesterday as Billboard's Best Artist under 21, beating hotshots Justin Bieber, 5 Seconds of Summer and Cody Simpson. The 17-year-old Kiwi singer celebrated with a trip to a Cleveland aquarium.
Lorde becomes acquainted with a bearded dragon at the Greater Cleveland Aquarium in Ohio.
• Congratulations to former Black Cap Jesse Ryder, 30, who scored a century to celebrate a two- year contract extension with English country club Essex.
• Congratulations to actress Danielle Cormack who was named this week as a ChildFund ambassador in Australia. The star of Wentworth, who has two sons Ethan and Te Ahi Ka, has been sponsoring Akullu, a young woman in Uganda, for nearly 10 years.
Wentworth star Danielle Cormack has this week been named a ChildFund ambassador in Australia.
Final four questions
1. Which high-flying London banker called a prominent Auckland real estate agent on Monday about buying two holiday mansions on Waiheke Island following the election result?
2. Which married realtor and which former politician are getting too close for comfort for some corners?
3. Which All Black is proving to be the team's cheeky documentarist, snapping his teammates in various states of sleep after a tough game of golf? Nice one, Beauden Barrett.
Beauden Barrett tweeted a picture of All Blacks taking a snooze on the bus.
4. His rumoured new girlfriend supermodel Naomi Campbell was nowhere to be seen, but where was Irish heart-throb Michael Fassbender snapped looking windswept and sporting a new moustache? On the Wellington to Picton Interislander ferry. The 38-year-old actor is in Marlborough shooting The Light Between Oceans.
Andree Malpass posted a picture to Instagram of herself with actor Michael Fassbender on the Picton ferry.
