Off The Wagon: Boozy Kelly Osbourne ‘Obnoxiously' Heckles Joan Rivers During Stand Up Show

Joan Rivers has had to deal with her fair share of hecklers over the span of her 55-year career.

But when it came to a sell-out crowd at the Saban Theater in Los Angeles on Jan. 17, the "Queen of Comedy" had one of her more "obnoxious" annoyances -- her E! Fashion Police cohost Kelly Osbourne.

Unashamedly boozing it up, 29-year-old Osbourne -- who last August admitted that she had gone to rehab seven times -- caused quite the ruckus in the audience, an eyewitness dished to Radar.

WATCH: Kelly Osbourne Explains Why She Got Drunk On The Plane

“Kelly had one drink before the show started and then during the show, she got up and went to the bar to get even more drinks,” the spy revealed.

As we've previously reported, while Osbourne's mom once had to put her in a "padded cell... to scare me," the ex-reality starlet does NOT claim to be sober still today.

“In the beginning of the show, there were open seats in the front row and Joan invited people in the 'bad' seats to come down front,” the source said.

“Kelly and her friend ran down from their seats that were further back, but Joan wouldn’t let her sit in the seats. She said ‘No, no one that I know.’ So Kelly and her friend ended up sitting a few rows back.”

As Rivers told her raunchy jokes to the crowd -- including one about Kim Kardashian's seven month-old daughter -- the recently single Osbourne began “heckling her,” the onlooker told Radar.

Said the source: “Kelly was yelling things out to Joan during the show when no one else was and she was throwing her hands up in the air. She was doing it obnoxiously. Everyone around her thought she was drunk.”