Lindsey Lohan was on "Letterman" last night . . . and during her interview, she and Dave prank called OPRAH WINFREY. Oprah is Lindsey's boss now, as Lindsay has a docu-series on OWN.

It wasn't much of a "prank call," but Dave did pretend to be Lindsay's secretary at first.  It's unclear how much of it was planned, but Oprah DID seem surprised.