Here's a classically trained singer and voice coach who evaluated several metal singers, and what she said about Ozzy without knowing who he was:

This is a singer with decent diction and good musical instincts but no command of vocal technique. He is massively over-adducting his vocal folds while driving enough air through them to get them to speak, but his throat is so tight that there is no flow or resonance. His rhythmic punctuation of the lyrics is very distracting, in contrast with Singer #1 (note: Bruce Dickinson) who delivered his text with rhythmic accents that served, rather than detracted from the flow of music and poetry. It hurt my throat so much to listen to him that I was tempted to ask Cosmo how long his career lasted before he either washed out or needed surgery. The entire range of his singing is contained within a single octave – with the exception of the moment when he yells “Oh Lord!” a little higher, in my opinion the only quasi-free vocal sound on the entire track.

https://www.invisibleoranges.com/ask-a-real-musician-5-classic-male-metal-singers/