"They cut to a backstage shot .... just as she put on the dress and I'm like: 'Oh, she's wearing it. Thank God,'" he recalled with a gasp and a chuckle.
O'Neill is creative director of Theia, an eveningwear and bridal collection from Montreal's J.S. Group. The label created the illuminated custom couture ball gown worn by the country star during Sunday night's Grammy telecast.
Much of the chatter about Underwood's performance hasn't been focused on her vocal chops but the key role her dress played in the performance. As the former "American Idol" champ belted out a ballad, the gown doubled as a screen where a series of digital projections were displayed.
A New York-based designer who originally hails from Ireland, O'Neill has a long professional relationship with Underwood dating back to his days designing for Badgley Mischka.
The label Theia — named for the Greek goddess of light — was established in 2009. Underwood has continued to support O'Neill and his new brand, wearing Theia designs to other high-profile fetes including the Country Music Awards.
Despite previous collaborations with Underwood and toiling with his team for days on the custom Grammy design, O'Neill said it still wasn't a sure bet Underwood would don the dress.
"Anything can go wrong backstage or on the day and Carrie may have not felt comfortable in the dress, and I don't know if there was a backup dress," said O'Neill. "You never know until you see the person in your dress that they're actually wearing it."
O'Neill was asked if he could make a "spectacular, very modern ball gown" for Underwood with a big skirt in fabric that would be suitable for a digital video projection — and he was given six days to do it.
O'Neill's daily train commute takes him over the Manhattan Bridge where the Freedom Tower is being constructed. He says the structure served as the inspiration for the dress design.
"The sun in the morning hits the tower in such a beautiful way ... it's sort of like a series of triangles that are broad at the bottom and are getting narrower and narrower as the tower has gotten taller, and the sunlight just hits it so perfectly every morning," he said.
The platinum ball gown was fashioned from duchess satin — as well as tulle and crinoline to support the skirt. It took over 80 hours of work by the Theia team.
O'Neill said he had two seamstresses sewing the fabric and one woman who spent close to three days adorning the inside corset of the dress with crystals.
The designer had no idea what to expect when Underwood finally stepped onstage, but said he got "goosebumps" when he saw the first swirls start to appear on her skirt.
"I think what was amazing was as a performer, she kept the focus of her performance entirely on her and her voice. So you were mesmerized by what was happening, which was happening on her as opposed to something happening of the theatre around her," he said.
"It was just a magical blend of fashion and art and technology and her voice."