Frances McDormand won best actress for Three Billboards Outside Ebbing, Missouri, and persuaded every female nominee to stand with her in a night full of statements about inclusion.
Britain's Gary Oldman was named best actor for playing Winston Churchill in World War Two epic Darkest Hour.
The Shape of Water won the most Oscars with four, including best film.
In her acceptance speech, McDormand put her Oscar on the floor in front of her and addressed executives as the female nominees got to their feet. It was one of the most powerful and symbolic moments of the night.
"Look around, ladies and gentlemen, because we all have stories to tell and projects we need financed," she said.
"Don't talk to us about it at the parties tonight - invite us into your office in a couple of days, or you can come to ours, whatever suits you best - and we'll tell you all about them."
Her request came amid a push for greater equality in film - especially in the wake of the Hollywood sexual harassment scandal.
She finished her speech with the words: "I have two words to leave you with tonight - inclusion rider."
Backstage, McDormand explained that an inclusion rider is a clause that actors can put in their contracts to insist on at least 50% diversity in the film's cast and crew.
"We're not going back," she said in the winners' room.
"This whole idea of women trending? No. No trending. African Americans trending? No. No trending. It changes now, and I think the inclusion rider will have something to do with that."
McDormand received a rousing reception for her best actress win, which came for playing a vengeance-seeking mother who is let down by the authorities after her daughter is raped and murdered.
It's her second Oscar, 21 years after her first for Fargo.
Jennifer Lawrence presented her with the best actress award and introduced the category by saying: "It's a new day in Hollywood, with new challenges ahead for all of us.
"But none of us will ever forget those who came before us - those who blazed a trail for my generation and those to come."