The BEPs kicked off the party with a performance of their latest single “I’ve Got a Feelin’” while Winfrey stood by onstage.

At first it appeared the estimated crowd of more than 20,000 was less than impressed as they stood motionless during the first few bars of the song. Then one fan started dancing, followed by a few more and a few more until the entire audience was dancing – and doing the same moves.

It turns out it was a flash mob event – the largest ever carried out as a matter of fact – orchestrated by Peas frontman and some choreographer friends who sent out a call on Twitter and Facebook for Oprah fans who could dance.

The response was overwhelming, with 800 hoofers responding to the request. Those dancers were taught the routine and then sent out to teach it to the crowd that showed up for the taping on Tuesday.

The result is incredible. (Plus it looks like it was a blast.)

Although it was his intention to stun Oprah, was also impressed.

“I didn’t think it was going to be this spectacular. This is the best performance I’ve ever had in my life, honestly. You talk about crowd participation, this is beyond… This is incredible,” the singer told Winfrey.

Oprah’s response? At first, “I just came out and I’m thinking, ‘How did everybody…? Did we hire all these dancers?'” And after it was over, “That is so cool. That is the coolest thing ever… Chicago, I love you.”

Take that North Korea.