Downey played Lewis Strauss, the former chairman of the US Atomic Energy Commission who mounted a behind-the-scenes campaign to strip J. Robert Oppenheimer of his security clearance by tarring him as a communist. Strauss's efforts were later exposed during a congressional hearing, as he sought confirmation to serve as President Dwight Eisenhower's commerce secretary.
Critics praised Downey for playing against his conventional type. He was considered a frontrunner, having collected Golden Globe and Screen Actors Guild trophies for his co-staring role.
The US actor earned his first Oscar nomination for playing Charlie Chaplin in 1992's Chaplin. After battles with scandal and addiction, he earned a second supporting-actor nomination for his role in the war satire Tropic Thunder.
Earlier Da'Vine Joy Randolph won the best supporting actress Oscar for her role as the grieving mother Mary Lamb in the boarding school drama The Holdovers.
While it's the first Oscar for the Black American actress, Randolph has dominated with a Golden Globe, a Critics Choice Award, a BAFTA and a SAG award for her role as the cafeteria manager at the New England school.
The drama, directed by Alexander Payne and distributed by Focus Features, sees three people thrown together at Barton Academy during the Christmas holidays in 1970.
Da'Vine Joy Randolph won the best supporting actress Oscar for The Holdovers. (AP PHOTO)
Mary Lamb, who is mourning the death of her son in Vietnam, is joined by a cranky teacher played by Paul Giamatti and a troublemaking student, played by Dominic Sessa.
The 37-year-old Yale School of Drama alumni, who got her start in theatre, has said she is grateful to hear from audiences who resonate with Mary's sense of loss and apathy toward the holidays.
"Imagine how hard it is if you're going through something and all that is on your TV is cheery, cheery, cheery, and you don't feel like that on the inside," Randolph told Reuters.
Meanwhile Britain's The Zone of Interest, about a German officer's family living next door to the Auschwitz extermination camp during World War II, collected the Oscar for best international feature film.
Prior to the awards, hundreds of pro-Palestinian protesters angered by the Israel-Gaza conflict shouted and slowed traffic in the blocks surrounding the Dolby Theatre in Hollywood. "While you're watching, bombs are dropping," one sign read.
"The Oscars are happening down the road while people are being murdered, killed, bombed," said 38-year-old business owner Zinab Nassrou.
Protesters hold letters and symbols reading "Eyes on Rafah," during a demonstration in Los Angeles. (AP PHOTO)
Oppenheimer, the three-hour drama directed by Christopher Nolan, led the field with 13 nominations. The movie is the frontrunner to win the prestigious best picture prize, capping its sweep of other major awards this year.
"If the best picture isn't Oppenheimer, it will be one of the biggest upsets, if not the biggest upset, in the history of the Oscars," said Scott Feinberg, executive editor for awards at The Hollywood Reporter.
After 2023 was marred by actors and writers strikes, the Oscars give Hollywood a chance to celebrate two global hits. Oppenheimer and feminist doll adventure Barbie, another best picture nominee, brought in a combined $US2.4 billion ($A3.6 billion) in a summer box office battle dubbed "Barbenheimer".
Oscar producers said they have planned unannounced cameos and other surprises to entertain audiences at home.
"My biggest hope is that they go through a range of emotions with us, that they feel happiness and joy, that we maybe make them shed a tear," executive producer Raj Kapoor said. "And then they somehow feel connected and inspired to also live their dreams."
For Nolan, the night could bring his first directing Oscar, as well as the award for adapted screenplay. The director of The Dark Knight trilogy, Inception and other acclaimed films has never had a movie win best picture.
The ceremony may end with "the industry-wide coronation for Christopher Nolan," Feinberg said. With Oppenheimer, "he has he has made his best possible argument yet for why he is worthy of this recognition."
with Reuters
