When Sheeran's tour was announced, O'Leary said their manager made a passing comment that Drax Project would love to open for Sheeran. He did not think it would become a reality.
He learned of the news after getting off stage during the Flochella Music Festival in Rotorua and was blown away.
O'Leary said everyone in the band were fans of Sheeran and couldn't believe they were about to perform their biggest gig of their career.
"He's a pretty amazing songwriter," O'Leary said.
"It will be by far the most people we've ever played to."
O'Leary, 23, said he believed 47,000 tickets had been sold to Sheeran's concert and even if half of those attendees were there for their performance, that would still be their biggest audience.
Drax Project's biggest crowd so far was around 3000-5000 people, he said.
"We've done school balls in the past, we've played to younger audiences but I feel Ed Sheeran has just got such a wide demographic."
O'Leary said they had not yet decided what their set would include as the selection was still sinking in.
"We haven't even thought about what we're going to play.
"I'm kind of getting a little bit more nervous the more we talk about.
"I'd say it's more exciting than anything."
He had no expectations of being able to meet Sheeran when he performs before him next month but he was hopeful and would love to be able to jam with him.
"He seems real down to earth."
O'Leary left Waihi Beach at the end of high school to study music in Wellington, where he was now based. He said he met Beachen and Thomson through Wellington's music scene and was asked to perform with Drax Project at a gig.
"I kind of just knew them from around the place.
"I got asked to fill in."
He said from there, he played more gigs and eventually joined the band as a permanent member.
"I don't think it was actually intended that I be in the band," he joked.
He said he was excited about the attention the band was receiving, especially with Woke Up Late, which he said was not even on the list of potential singles.
"It was a little song in the background."