The pair's chemistry on stage was undeniable.
Mark Ronson -- who co-wrote A Star Is Born's "Shallow" with Gaga, Anthony Rossomando, and Andrew Wyatt -- opened up about the headline-making Oscars moment during his appearance on The Ellen DeGeneres Show on Tuesday, explaining that Cooper had a "specific vision" for the performance.
"I think it was Bradley's idea," Ronson said of Cooper and Gaga emerging from the audience and performing the song with a single piano on stage. "I think he had a specific vision. I heard Lady Gaga talking about it."
"That might be the only time they performed the song together, so it made it feel really special," he added. "He's not a singer, so it was so brave of him to go and do that."
While Cooper and Gaga didn't win awards for their Star Is Born performances during Sunday night's ceremony, Ronson, Gaga, Rossomando and Wyatt took home the Oscar for Best Original Song.
"Making music, GRAMMYs is the epitome of what you shoot for, so this is so much from another universe. It's just another level, and what that song. I've worked with Lady Gaga before on Joanne, and we went and wrote this song," Ronson shared. "When I first saw the trailer for A Star Is Born and the way they make the song seem so exciting. And then when I saw the movie and the way they make that scene in the parking lot, I was like, 'Wow. They're really going to give this song a chance to get into people's hearts, because this movie's so great.'"
Ronson said he and Gaga wrote "Shallow" in 'a couple days."
"She had the first part. She had this idea for the chorus, she sat down at the piano, and she said -- she just kind of picked a few chords. She's like, 'I'm off the deep end,' or something, and then we all hit the ground running. She's super fast. And then I think we got the 'in the shallow' part," he recalled. "I said, 'We should do something a little weird. I know this is for this film, And it's like a ballad, but do your weird Gaga thing. So she took the 'shallow' part [and made it what it is now]."
A source opened up to ET on Tuesday about Gaga and Cooper's special bond.
"They are both very hard-working, driven people and during the making of the film, had a very professional work relationship," the source said. "From the moment they began collaborating with each other, people on the set saw how connected they were, which is why everyone wants their relationship to be real. There is endless chemistry between them.”
See more in the video below.