Kelly praised her parents for their solid union and posted a video featuring images of Sharon and Ozzy on Twitter.
In an accompanying post, she writes, "Happy anniversary mum and dad! you are the definition of true love!"
She then posted a link to the video clip alongside the words, "This made me cry!"
But Osbourne struggled to remember what anniversary it was for the Black Sabbath rocker and her mum; after writing, "30 years of true love!", she later corrected herself, "Oops it's 29 years."
Meanwhile, her brother Jack also marked the anniversary, posting, "Happy birthday america! Happy 29th anniversary mum and dad! I think they got married on the 4th for free fireworks."