Olivia Rodrigo surprised fans Friday night during her epic Lollapalooza Chicago debut … bringing out Weezer to perform 2 of their hits!

Check it out … she had fans cheering for the collab as they performed the rock band’s songs “Buddy Holly” and “Say It Ain’t So.”

It was a full circle moment for OR … who gushed to thrilled fans that Weezer headlined the first-ever concert she attended.

Olivia herself was the main act for Lollapalooza Night 2 … following sets from Bleachers, T-Pain, Korn, Djo (Joe Keery) and more.

Some of the hottest artists have yet to take the stage — Rüfüs Du Sol and Twice are Saturday’s main acts … while Sabrina Carpenter and A$AP Rocky are set to close out the 4-day festival on Sunday.