"The Revenant", which was widely expected to clinch the best picture trophy, lost it to "Spotlight" but earned its lead star Leonardo DiCaprio, 41, the best actor Oscar, two decades after his first nomination.
Its helmer Alejandro G Inarritu made history by becoming the first director since 1950 to repeat his win at Oscars. He won last year for "The Birdman".
The win for Tom McCarthy's "Spotlight", about the expose of sex child scandal in Catholic church by the spotlight team of the Boston Globe paper, was clearly one of the biggest upsets at the otherwise predictable ceremony.
"This film gave a voice to survivors. And this Oscar amplifies that voice, which we hope will become a choir that will resonate all the way to the Vatican," co-producer Michael Sugar said. McCarthy and Josh Singer also won the original screenplay.
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"We made this film for all the journalists who have and continue to hold the powerful accountable and for the survivors whose courage and will to overcome is really an inspiration to all," the director said.
"Climate change is real. It is happening right now. It is the most urgent threat facing our entire species, and we need to work collectively together and stop procrastinating... Let us not take this planet for granted," he said.
The Academy voters spread their acting awards to four different films. Brie Larson bagged the trophy for her moving portrayal of a woman kidnapped and kept in a garden shade in "Room" for years.
"I want to start big because the thing that I love about moviemaking is how many people it takes to make it," she said as she rushed through a list of 'thank yous'.
George Miller may have lost the best director trophy to Inarritu but "Mad Max: Fury Road" swept in technical categories with six wins in editing, production design, sound editing, sound mixing, costume and makeup & hairstyle.
Diversity was the recurring theme at the event with Rock not only addressing the racial discrimination in Hollywood but also talking about police brutality against blacks.
"Is Hollywood racist? You know, you have to go at that the right way. It is a different type of racist. You are damn right Hollywood's racist. Hollywood is sorority racist. But things are changing," Rock said, adding, "If they nominated hosts, I wouldn't even get this job.