EXCLUSIVE

Here's the scariest thing we've seen this Halloween -- O.J. Simpson.

Oh, also he's dressed up as a gynecologist ... which is way scarier than run of the mill O.J..

The ex-con spent his Halloween evening at the Blue Martini Lounge in Las Vegas, Nevada -- one of his fave haunts -- dressed up as an OB/GYN ... with the name Dr. Seymour Bush written on his doctor's coat lapel.

GET IT? SEYMOUR BUSH!!! Juice got jokes, y'all.

As usual, there were no lack of fans lining up to take photos with O.J., despite ... you know.

As far as we know, no one was stabbed to death at the bar that night. Only 2 questions remain: Where did O.J. get that jacket, and has anyone seen the real Dr. Bush?