So what the fuck are you, Ozzy? This is the exact question that a group of researchers at a leading genomics company set out to answer, because seriously, that man should not be alive. What they found was that Ozzy was genetically engineered for the rock 'n' roll lifestyle; basically, he's a real-life X-Men whose superpower is getting shitfaced.
Variations of certain genes allow for greater tolerance of (and dependence on) certain drugs. In Ozzy's case, that would be "all of them" -- his DNA contains genes that allow him to drink more, snort more, and inject more than us mere mortals. Ozzy's variation of the ADH4 gene, which is associated with alcohol, is something that these genomics experts had "never seen before." He's six times more likely to be addicted to alcohol, but also has a higher ability to absorb "marijuana, opiates, and methamphetamines."
And rodent heads.
This explains why Ozzy is still around while the likes of Keith Moon and John Bonham (not exactly lightweights) overdosed from what in the Osbourne household would be considered a light breakfast.