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."
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