In a recent interview with SiriusXM Ozzy Osbourne talks about Black Sabbath Dio albums. Ozzy says Ronnie James Dio did a "Good job" when performing with Black Sabbath. Ozzy also says he never really listened to the albums with Ronnie James Dio.
Dio first joined Black Sabbath in 1979 and sang on the 1980 album Heaven and Hell and the 1981 album Mob Rules. He left in 1982 but returned in 1992 for the album Dehumanizer. Dio again left and returned again with Sabbath from 2006 to 2010.
It was rough for Ozzy to be kicked out of the band, he co-founded. "At the time I was fucking sad because ... they were the only thing that ever really happened to me," Ozzy said.
Ozzy Osbourne Merchandise
Osbourne spoke about Black Sabbath "[getting] somebody completely different" other than getting another "Ozzy sound-alikes". Ozzy did like and respect Dio but he never really listened to the albums "It's like my ex-wife ... [when] you leave a band like that, it's just like getting divorced," said Ozzy. "You don't go, 'How's your new bloke? Is he better than me?'"
In a Kerrang! interview Iommi praised the band for getting things together after Ozzy Osbourne. "Ozzy was great, but Ronnie was a different singer altogether," Iommi said. "We wanted somebody who was different; we didn’t want to bring somebody in who was gonna sound similar to Ozz. So it was good to have somebody totally different, and Ronnie’s voice and the way he approached the songs allowed us to be able to try different things in a different way than what we’d done before. It opened up a lot more variety for us, really."
Both of the frontmen did a great job with Black Sabbath, I would even listen to an album where both of them contributed to the band at the same time. But that's for another universe.