"We don't know what's going to happen," he told Sharon Osbourne.
He wasn't joking. One surprise guest turned out to be Lady Gaga, who teased her appearance just hours before the free show, which was streamed live by AOL.
"Give it up for this wonderful man who's inspired people for generations," Gaga said while taking the stage in a blue dress and Dolly Parton-esque hair. "Wonderful Crazy Night is a beautiful record. And thank you so much for the Elton John AIDS Foundation. We all appreciate it so much, E."
John, 68, told Osbourne the concert was his husband's idea – a way of giving back to the city of West Hollywood, who he said had been "so amazing" to the couple, who host their annual Oscars bash in the neighborhood.
"And so we're doing it to a) say thank you to them; b) because it's a cool thing to do," John told Osbourne.
Gaga's appearance alongside John came one day before she could join him in the ranks of Oscar-winning pop stars (and soon after the pair announced they were collaborating on Gaga's next album").
The Born This Way singer is nominated for Best Original Song at the Academy Awards, for "'Til It Happens to You," which she wrote with Diane Warren for the documentary The Hunting Ground, about sexual assaults on school campuses.
Reporting by PATRICK GOMEZ