Kelly Osbourne was a barbarian who let her dog crap and pee all over her swanky WeHo condo ... and had the cajones to sue for her security deposit -- according to her ex-landlord who's now suing her right back.
Arshia Refoua claims he rightfully kept Osbourne's $18,700 deposit when she moved out in April 2013 ... because she left the place a dump -- holes in the walls, destroyed hardwood floors and dog poop and piss everywhere.
He also claims she caused extensive damage when she left the bathtub running ... flooding her condo and the ones under her.
We broke the story ... Osbourne fired the first shot by suing Refoua to get back the deposit -- and denied all pigsty allegations.
Now she could lose way more than the $18K -- Refoua's suing her for damages of at least $50,000.