Usher made a guest appearance at the Black Eyed Peas’ Super Bowl halftime show in 2011, and bandleader will.i.am says he was happy to return the favor Sunday.

Will.i.am performed “OMG” — which he wrote and produced — alongside Usher at both events. 

“How can I explain it? There’s no words to really explain that,” he told The Hollywood Reporter after the performance. “Now 13 years later, it means something totally different. It’s beautiful. It was wonderful. It’s fantastic.”

As they performed “OMG,” roller skaters danced and flipped onstage. “The reason I say our section was the most ‘What the F?’ is because we’ve never seen roller skating at any type of big world stage like that. It felt like the urban Olympics. He brought the urban Olympics to the Super Bowl.”

“There were a lot of OMG moments in the show. There were so many OMG moments. That 60 seconds of ‘OMG’ was all you needed because the show was ‘oh my gosh,’” he adds.

Another favorite moment? “That one time when he chest naked onstage,” he said, laughing. “That was crazy.”

Usher and will.i.am at the 2011 Super Bowl.

Kevin Mazur/Getty Images

Usher’s performance also featured Alicia Keys, Ludacris, Lil Jon, H.E.R. and Jermaine Dupri, and the Grammy-winning star ran through hits like “U Got It Bad,” “Love in this Club,” “U Don’t Have to Call” and “Yeah!”

Usher did so well that will.i.am didn’t realize the singer was 45 years old: “Damn, I didn’t know Usher was 45. I thought he was at least 33.”

The dance-flavored “OMG” was released in 2010, topped the all-genre Billboard Hot 100 chart for four weeks and reached 8x-platinum status, which is equivalent to selling 8 million singles.

Will.i.am says when he composed the song he knew it “was going to be a smash.” He said he created the song after the success of the Black Eyed Peas’ “I Gotta Feeling,” which features the lyrics: “Go out and smash it, like, ‘Oh my god.’”

“‘OMG’ is because of ‘I Gotta Feeling,’” he explained. “‘I Gotta Feeling’ was the ingredient. It was the DNA.

“Working with Usher has been awesome,” he added. “I want to work more with him in the future. I’m really proud of his [new] album Coming Home. It’s amazing. I’m proud of his performance now.”