I honestly wouldn't want to see Jack Black as Ozzie... unless it was going to be a comedy... I have a hard time taking drama seriously when it's being played by someone that I laugh at... and Jack Black is starting to get slightly annoying to me... Johnny Depp would do the character much more justice, in my opinion. I know Jack Black is a good actor as well as a comedian. But, Johnny is an infinitely better actor... (see: Blow, Donnie Brasco, The Libertine, Finding Neverland, Secret Window, From Hell, Sleepy Hollow, hell... even Edward Scissorhands).
Colin Farrel is good too.... but I can't see him pulling off Ozzy at all. He's too pretty faced to play a metal God.