Leonardo DiCaprio is widely expected to take home his first best actor Oscar later.
The Revenant, which he is nominated for, is also seen as a strong contender for best picture and best director.
The ceremony gets under way at the Dolby Theatre in Los Angeles at 17:30 PST (01:30 GMT).
But it is being boycotted by some Hollywood figures who are protesting about the lack of ethnic diversity among this year’s nominees.
All 20 nominees in the best acting or supporting acting categories are white.
Last week, Harvey Weinstein discouraged a boycott, reassuring those concerned that host Chris Rock would “annihilate” Hollywood in his opening monologue at the ceremony.
The black comedian is known for dealing with racial issues in his routines and it is expected he will make reference to the recent controversy.
Earlier this month, director Spike Lee – who was one of the first figures to speak publicly about the lack of diversity – said the boycott had been vindicated by the response from organisers.
The Academy, which hands out the Oscar honours, announced it would double the number of women and ethnic minority members by 2020 after the protests.
The Oscars take place two weeks after DiCaprio won a Bafta for best actor – often a good indicator of Academy Awards success.
DiCaprio has never won a best actor Oscar before, despite three previous nominations and relentless campaigning from fans.
The 41-year-old is up against Eddie Redmayne, Bryan Cranston, Matt Damon and Michael Fassbender for the statue.
The Revenant faces tough competition in the best picture category from The Big Short and Spotlight.
The Big Short was not initially seen as the favourite to triumph, but its chances were boosted after it won the top prize at the Producers Guild Awards (PGA).
The best picture winner at the PGAs has gone on win the same award at the Oscars every year for the past eight years.
Brie Larson is up for best actress alongside Saoirse Ronan, Charlotte Rampling, previous winners Cate Blanchett and Jennifer Lawrence – the latter has already picked up a Golden Globe for her performance in Joy.
Kate Winslet stands a strong chance of taking home best supporting actress after winning in the category both at the Golden Globes and the Baftas.
She is up against Jennifer Jason Leigh, Rooney Mara, Alicia Vikander and Rachel McAdams.
The nominees for best supporting actor are Christian Bale, Mark Ruffalo, Sylvester Stallone, Tom Hardy and Mark Rylance.