Sally Field -- Guess What's Under My Oscar Dress?

If you see Sally Field at the Oscars Sunday ... feel sympathy she's about to frickin' burst.

Sally wants a perfect hourglass figure when she hits the red carpet ... but it won't be from exercise -- she'll be wearing industrial strength underwear.

We're told the 2-time Oscar winner popped into Trashy Lingerie in West Hollywood ... looking for something to hold it all together underneath her Academy Award gown.

She saw a nude-colored corset and plunked down $200.

Apparently she liked it -- she really liked it.