"Oppenheimer" received the top honor at the Screen Actors Guild (SAG) Awards on February 24.
Historically, the SAG award is a strong Oscar bid and makes Oppenheimer a strong favorite for best picture, AFP reported.
Christopher Nolan's epic drama about the father of the atomic bomb also took home awards for best actor for Cillian Murphy, who plays the titular scientist, and best supporting actor for Robert Downey Jr., who plays his bitter rival.
"Thank you for inviting me to be a real part of the making of this tremendously important film," said Kenneth Branagh, speaking on behalf of the cast.
The mood at the gala was celebratory as members of SAG-AFTRA — the union that represents about 120,000 performers, including A-list stars — gathered in large numbers for the first time since the strike ended in November.
Branagh recalled how the film's cast walked out of the London premiere last July when the work stoppage was about to begin.
Ultimately, the guild won its terms, including better pay and protection against artificial intelligence (AI) from studios including Disney and Netflix./BGNES