GARTH BROOKS showed up at the Oklahoma State University graduation ceremony the other night.  But he wasn't there to perform... or address the graduates.  He was picking up his Master's Degree.

This wasn't an honorary pat on the head either... Garth actually studied and earned it.  I'm guessing he did it online, or through a correspondence course-type thing because no one seemed to know he what he was up to.

Everyone at the ceremony was surprised to see him, though... and they Tweeted like crazy.

Student @Embreyp wrote, "Yeah that's right I got my MS with Garth Brooks!"

@Sara_Ramsey, who was a guest in the audience, said, "Brother just got his Master's from Oklahoma State.  So did Garth Brooks.  Sadly, Garth got more applause."

Another attendee @bryankarns tweeted, "#OKState Boring Friday night graduation just got interesting... Garth Brooks walked across the stage to get his MBA."

And a parent of a graduating student, @LimiGal, said that she "watched Garth Brooks get his Master's degree at OK State Univ along with my daughter."

(Check out some photos of Garth and his fellow O.K. State graduates.)

(By the way, Garth got his Bachelor's degree in advertising from Oklahoma State way back in 1985.  But you already knew that.)