The British star, 59, picked up the best actor prize for his turn in Darkest Hour, while Frances McDormand won the best actress gong for Three Billboards Outside Ebbing, Missouri.
The Shape Of Water picked up four of the 13 Oscars it was nominated for, including best director for Guillermo del Toro and the night’s biggest prize, best picture.
Get Out director Jordan Peele became the first black writer to win the best original screenplay for his social satire, as British cinematographer Roger Deakins finally clinched a gold statue for his work on Blade Runner 2049 on his 14th nomination.
The Time’s Up and Me Too movements, prompted by the Harvey Weinstein sexual harassment scandal, were given the spotlight during a moving segment of the ceremony, while Salma Hayek, Ashley Judd and Annabella Sciorra, e all Weinstein accusers, appeared on stage together.
Disgraced mogul Weinstein has been accused of sexual harassment and assault by scores of actresses. He denies all allegations of non-consensual sex.
Sam Rockwell, Jane Fonda, Patrick Stewart and Steven Spielberg were among the attendees to sport Time’s Up pins on their evening wear.
Rockwell picked up the best supporting actor Oscar for Three Billboards Outside Ebbing, Missouri, while Allison Janney won the best supporting actress prize for I, Tonya.
Warren Beatty and Faye Dunaway returned to present the final award of the night a year after the best picture fiasco when they mistakenly announced La La Land as the winner rather than Moonlight.
Host Jimmy Kimmel introduced the pair saying: “This is the home stretch, nothing could possibly go wrong.”
Oldman appeared emotional as he accepted his award and in a lengthy speech, he thanked “the Academy and its members for this glorious prize” as well as his colleagues, wife and family. He thanked America “for the many wonderful gifts it has given me – my home, my livelihood, my family and now Oscar.”
The actor also sent out a special thank you to his mother.
He said: “I would like to thank my mother, who is older than the Oscars, she is 99 years young next birthday and she’s watching the ceremony from the comfort of her sofa.
“I say to my mother, thank you for your love and support, put the kettle on – I’m bringing Oscar home.”
Kimmel addressed the Weinstein scandal as he opened the 90th Academy Awards.
He said: “What happened with Harvey and what is happening all over was long overdue.
“We can’t let bad behaviour slide anymore, the world is watching us and we need to set an example and the truth is if we are successful and if we can work together to stop sexual harassment in the workplace, if we can do that, women will only have to deal with harassment all the time at every other place they go.”