Mark O'Brien plays Carter Mitchum in Marriage Story. The movie is nominated for six Oscars, including best picture, best actor and best actress. (CBC)

St. John's actor Mark O'Brien has come a long way since playing Des Courtney on Republic Of Doyle, shot in his hometown. Now, he's celebrating his recent film being nominated for six Academy Awards.

"I don't believe that it's happening," O'Brien said.

O'Brien stars alongside Adam Driver and Scarlett Johansson in Marriage Story, telling the story of a husband and wife struggling through divorce and keeping their family together. O'Brien plays Carter Mitchum.

"It was wild," O'Brien said.

"I had been a fan of Noah Baumbach, the writer, director, for years and years. He's done some really amazing films. When I found out I was like, 'Oh! I thought they didn't like me!'"

Marriage Story shows an incisive and compassionate portrait of a marriage breaking up, and a family staying together. (TIFF)

Marriage Story is nominated for six Academy Awards including best picture, best original music and screenplay, best supporting actress, and best actor and actress to Driver and Johansson respectively. O'Brien said working with some of the most notable talent in Hollywood was exciting.

"You kind of forget, you're like, 'Oh yeah, there's Kylo Ren, and [Black Widow] from The Avengers,'" O'Brien said.

"On top of it they're great actors. And I got to say, full honesty, they're the nicest people in the world. Very normal. Adam has a son around the same age as my daughter. They're such big stars, but we would just talk about not sleeping because you have a kid. I was like 'Oh! I guess they're the same.'"

The 92nd Academy Awards will take place Sunday in Los Angeles.

I just really love working with people I admire. - Mark O'Brien

O'Brien's star continues to get brighter. In the years since entertaining Canadians on Republic of Doyle, he has found himself in big-budget movies and television shows, including Halt and Catch Fire, and Arrival — which was nominated for best picture at the Oscars in 2017.

"I just really love working with people I admire, people who I think are good at what they do and [are] nice people," O'Brien said.

"Any opportunity I get to be in, like, Marriage Story or Arrival, which helped me a lot or Halt and Catch Fire … it helps a lot in the sense that, I guess, people can trust you a little more, and that means they can trust you to work with these big people or really experienced people."

O'Brien was especially excited to work with Marriage Story director Noah Baumbach.

"It was cool for me to be in the world of a Noah Baumbach movie," O'Brien said.

"He has this kind of … repertory thing going on where a lot of people reoccur in his movies. And so the fact that I was just in one of them, I just thought was really cool. It was a real treat in that kind of way."

O'Brien is unable to attend the award show Sunday night due to filming in New York. He plans to watch the show with his cat.

"I'll be watching by myself with a Marriage Story banner, cheering on."

He has his eyes on other projects, including his first film as a director. The Righteous was written by, directed and stars O'Brien, and was filmed in St. John's in 2019.