Eilish previously won for 'No Time to Die' in 2022.
Billie Eilish and Finneas O'Connell are now two-time Academy Award winners after taking home the award on Sunday for "What Was I Made For?" from Barbie.
During the broadcast, Ariana Grande and Cynthia Erivo took to the stage to present Best Original Score and Best Original Song -- the category that brought the siblings up for their second time in five years.
"I had a nightmare about this last night," Eilish said as she approached the microphone after hugging Barbie's America Ferrera, Greta Gerwig and Margot Robbie in the crowd. "Thank you so much to the Academy, I just didn't think this would happen, I was not expecting this."
"Thank you to Greta, where did you go, I love you," Eilish -- who previously won in 2021 for "No Time to Die" from the eponymous James Bond film -- said. "This goes out to everyone who was affected by the movie and how incredible it is," she said.
Earlier in the night, the pair moved audience members to tears with their rendition of the song -- which also won a GRAMMY last month. The nine-time GRAMMY-winning singer serenaded the audience -- and fans around the world -- with the song.
Talking with ET from the carpet, they teased their performance, calling it "super simple" but also very meaningful.
"What I like about it is that it's very back to how we created the song, very much just me and Phinneas and kind of nothing else," she said. "It's very much the two of us that's really special to me."
The 2024 Academy Awards, hosted by Jimmy Kimmel, airs live from the Dolby Theatre in Los Angeles on Sunday, March 10, at 4 p.m. PT/7 p.m. ET on ABC. Follow along at ETonline.com for full Oscars coverage, including red carpet arrivals, the complete winners list and more.
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