Dern took home the Oscar for best supporting actress for her role as a ruthless divorce lawyer in Marriage Story, and dedicated it to her celebrity parents Diane Ladd and Bruce Dern.

"This is the best birthday present ever," said Dern, who will turn 53 on Monday.

Pitt was named best supporting actor for playing a charming stunt double in Once Upon a Time in Hollywood after having collected an armful of trophies earlier this year for the role. It was his first acting Oscar.

"I'm a bit gobsmacked to tell you the truth," said Pitt, who recalled his early days starting out as an unknown actor.

"Once Upon a Time in Hollywood - ain't that the truth?," he said, dedicating the Academy Award to his six children with former wife Angelina Jolie.

Parasite got off to strong start by winning the original screenplay Oscar for a delighted Bong Joon Ho - the first of six potential Academy Awards for the film on Sunday.

The social satire about the wealth gap in South Korea is also seen as a front-runner for the coveted best picture prize, as well as international feature and best director for Bong. No film in a foreign language has ever won best picture at the Oscars.

American Factory, about the decline of jobs in the industrial Midwest from former U.S. President Barack and first lady Michelle Obama's new production company, won for best documentary.

Nazi satire Jojo Rabbit brought a best adapted screenplay Oscar for director, star and writer Taika Waititi.

Read also: Brad Pitt wins supporting actor Oscar for 'Once upon a Time in Hollywood'

Despite again having no formal host, the Oscars ceremony started with a surprise appearance by former hosts Steve Martin and Chris Rock, who delivered a string of barbed jokes about the lack of female directors and people of color among this year's nominees.

"I thought there was something missing this year," said Martin.

"Vaginas?" quipped Rock, to loud applause.

Rock pointed out Cynthia Erivo, who played slavery-era freedom fighter Harriet Tubman in Harriet and who was the only actor of color nominated this year.

"Cynthia did such a great job hiding black people that the academy got her to hide all the black nominees," said Rock.

In another surprise, rapper Eminem took the Oscars stage to perform "Lose Yourself," his 2003 Oscar-winning song from the movie 8 Mile.

The nominees and winners are chosen by the 8,000 members of the Academy of Motion Picture Arts and Sciences.

The coveted best picture prize, to be announced at the end of the three-hour show, is thought to be a three-way race between Parasite, British director Sam Mendes' immersive World War One movie 1917 from Universal Pictures, and Once Upon a Time in Hollywood, Quentin Tarantino's love letter to show business, from Sony Pictures.

Dark comic book movie Joker from Warner Bros, which has a leading 11 nominations, Netflix divorce drama Marriage Story and the streaming service's mob epic The Irishman, race-car drama Ford v Ferrari from 20th Century Studios, and novel adaptation Little Women from Sony Pictures, round out the competition for the top prize.

Netflix boosted its Hollywood credentials by getting a leading 24 nominations this year. But the coveted best picture Oscar may elude it yet again on Sunday, awards watchers say.