He joined Black Sabbath co-founder Tony Iommi, Rolling Stones guitarist Ronnie Wood and other famous faces in paying tribute to the British heavy metal singer after his death on Tuesday.
Elton posted on Instagram: "So sad to hear the news of Ozzy Osbourne passing away.
"He was a dear friend and a huge trailblazer who secured his place in the pantheon of rock gods - a true legend.
"He was also one of the funniest people I've ever met. I will miss him dearly. To Sharon and the family, I send my condolences and love. Elton xx."
Black Sabbath co-founder Tony Iommi said it was "brilliant" to reunite with the band just weeks before Osbourne died, saying the singer "really wanted" to perform together again and "felt at home".
He told BBC Radio 4: "It is really a shock. It's really hit me today to be honest, but yeah, a terrible shock."
On their reunion concert, Iommi said: "It was good for all of us, and lovely to be able to get together for a final thing. I mean, we didn't realise it was going to be this final. We didn't expect him to... but he hasn't looked well for a while."
Wood joined Osbourne at Black Sabbath's last gig at Villa Park, Birmingham, earlier this month.
He posted on X: "I am so very sad to hear of the death of Ozzy Osbourne.
"What a lovely goodbye concert he had at Back To The Beginning in Birmingham."
Singer Yungblud, who sang Black Sabbath's Changes at the Back To The Beginning farewell concert on July 5, said he was "heartbroken" by the news and called Osbourne "the greatest of all time".
"I didn't think you would leave so soon the last time we met you were so full of life and your laugh filled up the room," he wrote in a social media post.
Motorhead said they had lost a "dear friend" in a post that recalled the band's late vocalist Lemmy's friendship with Osbourne.
"We lost our dear friend today today. Ozzy was a pioneer, a guiding force for all in hard rock and metal, and a great guy too.
"Lemmy and Ozzy were brothers in arms who enjoyed many adventures together, and his loss is seismic. RIP Ozzy, we will always love and celebrate you ...", they wrote.
AC/DC posted that Osbourne's death was a "great loss to all that loved him", with Metallica members calling him an "icon" and "mentor" while sharing photos from rock photographers Ross Halfin and Noah Abrams.
"He taught us how to play in the big leagues while at the same time being warm, welcoming, engaging, and all around brilliant," they wrote on Instagram.
Among the other stars paying tributes were David Beckham, Green Day's Billie Joe Armstrong, Queen guitarist Brian May and US movie star Adam Sandler, who said he felt "so happy" that he was able to meet Osbourne.
May said Osbourne's final concert this month "was a glorious way to say goodbye".
Aston Villa, Osbourne's home town football club, said it was "saddened" to learn of the star's passing.
"Growing up in Aston, not far from Villa Park, Ozzy always held a special connection to the club and the community he came from," the club said.
"The thoughts of everyone at Aston Villa are with his wife Sharon, his family, friends, and countless fans at this extremely difficult time."
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