Jamie Lee Curtis, Lily James and the cast of Japanese best picture nominee Drive My Car were among the first to arrive at the Dolby Theatre where 2500 gown- and tuxedo-clad attendees were expected for the 94th Academy Awards.
"I'm 63 years old and they still asked me to come to the party," Curtis told a reporter.
"I'm thrilled to be able to represent women of a certain age."
Curtis said she would deliver a tribute to acting legend Betty White, who died in December just shy of her 100th birthday.
A dark western, New Zealand director Jane Campion's The Power of the Dog, will battle feel-good deaf family drama CODA for the prestigious best picture prize.
A win by either movie would mark a milestone by handing the statuette to a streaming service.
Netflix released Power of the Dog - for which Australian actor Kodi Smit-McPhee has a best supporting actor nomination - while CODA streamed on Apple TV+.
Producers said they planned an upbeat show but will acknowledge the crisis in Ukraine, which has killed thousands and driven a quarter of Ukraine's 44 million people from their homes.
Actor Mila Kunis, who was born in Ukraine, is among the night's presenters.
After past complaints about a lack of diversity, two of this year's four acting accolades may go to people of colour.
Will Smith is likely to earn his first Oscar for King Richard, awards watchers say, while Ariana DeBose is a strong favourite in best supporting actress for West Side Story.
Comedians Amy Schumer, Regina Hall and Wanda Sykes will host, the first time three women have guided the ceremony.
In a bid to draw more viewers, especially younger ones, after years of declining ratings, the three-hour show starting at 11am AEDT will add two awards, the results of fan votes for favourite film and scene.
King Richard star Smith is the best-actor favourite for his role as the determined father of tennis legends Venus and Serena Williams. Competitors include Benedict Cumberbatch (The Power of the Dog) and Andrew Garfield for musical Tick, Tick ... Boom!.
Best actress is the most unpredictable. Many experts say Jessica Chastain's portrayal of TV evangelist Tammy Faye Bakker will triumph, though some give the edge to former Twilight vampire Kristen Stewart, who played Princess Diana in Spencer.
Nicole Kidman is also in the running, for her portrayal of US comedy legend Lucille Ball in Being the Ricardos.
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