Oprah Winfrey -- I'm Gonna Be a Whorehouse Madam in Richard Pryor Movie

Oprah Winfrey's been cast to play Richard Pryor's grandmother -- a cruel and abusive lady pimp -- in the upcoming biopic directed by O's pal Lee Daniels.

Pryor's widow, Jennifer, tells TMZ ... Oprah got hold of the script, and was desperate to play Marie Carter -- the woman who raised Pryor in her brothel in Peoria, Illinois ... and who was known to beat him. As we told you ... Mike Epps scored the Pryor role.

We're told Oprah really wanted to play Grandma Carter because she's anxious to show her gritty side -- after her 'Butler' performance -- and let's face it ... there's not much grittier than an abusive grandma running a whorehouse while raising a kid.