Ozzy Osbourne is another unique case. To me, his voice has kind of a vaudevillian character to it. It's very theatrical and it projects well. For some reason, Ozzy sounds to me like a male version of Ethel Merman. Except instead of singing show tunes, he's singing heavy metal.
Ozzy really improved a tremendous amount as a singer. On the first Black Sabbath album, his pitch was all over the place. His voice was dropping clams everywhere.
He steadily improved in the second and third albums, and by the fourth album, he had developed his voice to be far more technically proficient.
I don't know for sure, but I believe that Ozzy is pretty much musically illiterate. I don't think he knows (or cares) what chords are being played while he's singing or has any idea at all about music theory, but he clearly has a good ear and good instincts. Plus, he just radiates charisma and stage presence.