Security for the ceremony was tight on Sunday as some of the biggest names in Hollywood walked a red carpet decorated with trees to evoke the feeling of a Zen garden.
Designers said they hoped the scene, which also adorned the stage, would deliver a feeling of calm in a chaotic world.
Organisers said they were working closely with the FBI and Los Angeles police after a federal warning of a possible Iranian threat against California, though authorities have cited no specific or credible danger to the Academy Awards.
Attendees had to cross through several traffic checkpoints and go through metal detectors to make their way to the Dolby Theatre.
Hosted by Conan O'Brien for a second year, the festivities will feature a wide-open contest led by Sinners with 16 nominations - a record number in the nearly 100-year-old history of the Oscars - as Hollywood grapples with geopolitical tensions, industry consolidation and anxiety over artificial intelligence.
The glitzy celebration, Hollywood's most over-the-top gala of the year, will take place as the US wages war on Iran.
O'Brien said he planned to touch on current events but his primary mission was to make people laugh and feel at ease.
The ceremony masks the unease in the film business over where movies are being made as studios chase tax incentives and lower costs elsewhere in the US and overseas, weakening Hollywood's grip on production.
This year's awards contest holds an unusually high potential for surprises.
The race for best actor is an especially unpredictable one, pitting Timothée Chalamet against Leonardo DiCaprio and Michael B Jordan.
Chalamet had been considered a frontrunner for his acclaimed performance as a ping-pong hustler in Marty Supreme, but his prospects seemed to dim over an awards-season campaign featuring a streetwear line and a giant blimp and remarks dismissing ballet and opera.
One Battle After Another, starring DiCaprio as a one-time political radical now parenting a teenager, was seen as the frontrunner for best picture after stacking up trophy after trophy at recent ceremonies.
But Sinners, a celebration of blues music and Black culture in the Segregation-era US South starring Jordan, made a late surge with a win this month at the Actor Awards.
Despite almost no campaigning, Sean Penn is viewed as the best supporting actor favourite, with Australia's Jacob Elordi also nominated in the category for his performance in Frankenstein.
That award, could easily also go to Stellan Skarsgård (Sentimental Value) or Delroy Lindo (Sinners).
Another Australian, Nick Cave, is nominated for the title song to Clint Bentley's film Train Dreams, starring Joel Edgerton.
Buckley is considered a lock for best actress for her portrayal of Agnes Hathaway, the wife of William Shakespeare, as the couple navigate the death of their 11-year-old son in Hamnet.
Australia's Byrne, who's hoping to win her first Oscar, arrived wearing a strapless black fishtail gown by Dior, with floral embroidery across the bodice and hem.
Awards experts say the rest of the top categories are up for grabs.
Winners of the gold Oscar statuettes are chosen by the roughly 10,000 actors, producers, directors and film craftspeople who make up the Academy of Motion Picture Arts and Sciences.
The Academy took steps this year to try to ensure voters have actually watched the movies they are voting on.
The online balloting system for the first time tracks whether a voter has streamed each movie. Voters, however, can check a box to say they watched the movie elsewhere outside the Academy website.
with Reuters
