The pop star went off on the Black Sabbath rocker Wednesday -- not long after Ozzy aired out his opinion on BS's habit of throwing up dance clips of herself at home ... where she's often twirling around and wearing hardly anything at all.

Britney starts out by defending Kate, writing ... "I adore Kate Beckinsale especially because she's from London. I realized how incredibly cruel people were talking about her IG saying she needs more age-appropriate content."

She goes on to say that she appreciates Kate's signature bow and how much she's telling haters to piss off -- and then pivots into how she herself has caught a lot of heat for her own videos ... which some label weird/offputting.

Britney goes on to explain why she dances so much -- she says she's good at it, and loves it -- and then takes a direct swipe at Ozzy and his family, adding ... "I'm gonna do a photoshoot with Kate and tell the Osbourne family who is the most boring family known to mankind to kindly f*** off!!!"

Of course, she's responding to Ozzy name-dropping Brit in a podcast he did with his brood -- where he said he was fed up with Britney's dance videos ... calling them nauseating.

It sounds like she doesn't care what Ozzy thinks though ... and she apparently even sees a little of herself in Kate. Future collab coming??? Sure sounds like Britney might want one.