A source tells Us Weekly that Osbourne rented out Beacher's Madhouse inside the Hollywood Roosevelt Hotel to help the 19-year-old celebrate her special day.
"Her mom Tish went, and so did her boyfriend Liam [Hemsworth]," the source says of Cyrus. "The group was up until 3 a.m. just dancing up a storm and having fun."
Cyrus seemed quite impressed with Osbourne's planning skills, calling it "the best party ever" on Twitter. "Let's just say there was a unicorn in the lobby!" (An onlooker tells E! News there was a horse with a horn attached to its head inside the soiree.)
Prior to Tuesday's birthday blowout, Osbourne teased that it was "going to be the sickout thing [Miley has] ever seen in her life."
"That was my mission for her, to make her birthday...the best day ever."
Osbourne and Cyrus first became friends while shooting the comedy So Undercover. The film has yet to be released.
