(gibson) Ozzy Osbourne had a pair of unlikely guest vocalists on his 1988 album No Rest for The Wicked: Howard Kaylan and Mark Volman, better known as Flo and Eddie from Frank Zappa's Mothers of Invention and The Turtles (of “Happy Together” fame).
In the second part of his long-ranging interview with Bravewords.com, bass player Bob Daisley said Flo and Eddie sang background vocals on the album, for which Daisley wrote the lyrics. No Rest was Ozzy's first album with Zakk Wylde on guitar.