The two pals hit up Ci Siamo -- a popular Manhattan restaurant ... Oprah in a gold dress under a long white coat and Gayle going with a more muted black and teal outfit, perfectly matching her scarf.

The two were joined by Megan Sullivan, general manager of the restaurant, who walked into the joint for what Gayle clearly thought was supposed to be a small dinner ... though Oprah had a big surprise to her.

Winfrey posted a clip from the restaurant to her Instagram Saturday ... walking in with a distracted Gayle -- when dozens of voices scream out "Surprise."

Gayle bounces back in total shock while Oprah laughs at her reaction ... explaining in the caption it's hard to surprise Gayle -- and, their group had to tell a ton of lies to the star.

The camera flips around to the room ... and check out the star power -- Tina Knowles, Robert De Niro and many more bold-faced names took the time to celebrate Gayle.

All of this was for Gayle's 70th ... which isn't until December 28th -- all part of the surprise we reckon.

Gayle and Oprah have been friends for decades ... first meeting back in 1976 at a local Baltimore TV station when Oprah was an anchor and Gayle got a job as a production assistant.