Fergie is in the hot seat on Oprah's Next Chapter.

The Black Eyed Peas songstress is opening up to the talk show host about everything ... from previous drug use to cheating rumors involving hubby Josh Duhamel.

After seeing this preview, it doesn't look like Lady O holds back when it comes to the questions.

We can't wait to watch Fergie spill the beans when the episode airs on October 21!