Just because Ozzy is retiring, doesn't mean Ozzfest is going away.
Ozzfest ran from 1996 to 2008 every single year, skipped 2009, and then came back in 2010. Then Ozzfest was solely held in Japan in both 2013 and 2015, and has since been back in the United States, billed as Ozzfest Meets Knotfest from 2015 to present day.
Ozzy recently announced his retirement from touring and farewell tour with Stone Sour, though stated that the retirement was strictly from massively long tours. Osbourne said he'd still be doing festivals, or as Sharon put it in the below press conference, "anything but Coachella."
In that same conference, Sharon also says Ozzfest "will still go on," which isn't too surprising given Ozzfest Meets Knotfest is a multiple-day thing and not a giant tour. So at this rate we're still due to see Ozzy up on stage at least once per year, or he might skip a few years. Ozzy did not perform at last year's Ozzfest.
Someone also asks Ozzy if his coming No More Tours 2 is "for real this time" in reference to his 1992 farewell tour titled No More Tours. Osbourne says the reason he was going to retire in 1992 was due to a misdiagnosis of Multiple Sclerosis, which seems like an incredibly valid reason to announce that you're hanging it up from tour.