But on December 1, O'Neill will perform at a fundraising dinner at Auckland's Langham Hotel for The Hearing House children's charity.
The 40-year-old is booked years in advance and will be spending up to 10 months of the year performing in the world's top opera houses.
But he longs to spend more time at home. "My dream is to do six months in New Zealand and six months on the road."
The journey home will also be a chance to see his wife, Carmel, and three children, 4-year-old twins Tom and Grace, and Violet, 1, who are based in the family's North Shore house.
He talks to his children on Skype every day, and the twins - surely contenders for the world's youngest Wagner fans - are already showing signs of musical promise, he says. As for following in his footsteps, O'Neill is not so sure, and thinks he may gently nudge them instead in the direction of their mother, a "hotshot lawyer".
It's important to the couple that the twins start school in New Zealand, and have a good old-fashioned Kiwi childhood like their parents.
O'Neill grew up in Ashburton, the oldest of three boys in an Irish Catholic family. His schoolteacher mother and freezing worker dad, both still living in Ashburton, raised their sons with a love of music. Young Simon played pipe organ at church, learned the piano and performed in brass bands, but didn't turn to singing until he was in his teens. Rejected by the University of Otago's music programme when he first applied, he was accepted the following year, and then went on to study at Victoria University in Wellington, before receiving scholarships to the Manhattan School of Music and the Juilliard Opera Centre in New York.
Since then his career trajectory has been unstoppable.
As well as at Covent Garden, O'Neill is a principal artist with New York's Metropolitan Opera and Milan's La Scala. In 2009, the UK's Daily Telegraph called him "the best heroic tenor to emerge over the past decade".
"My career's going bigger and better than I could have ever imagined," he says, but is quick to admit it's not an easy lifestyle.
"The international opera singing career is a very lonely career - you're by yourself, you turn up and sing the lead role and it's a great honour, but you're just by yourself. The life of a musician is pretty hard."
That life peaks for an opera singer from the age of 40 to 50, so O'Neill says he is ready for another decade of "going hard".
O'Neill would also love to perform more at home.
"Every year, at least once, I want to be in New Zealand singing for our country," he says. "I really have disliked the history of some New Zealand opera singers not returning to sing for our local people."
O'Neill will be having his wish granted in a few weeks, after going to Japan to sing Tosca, when he performs at the Hearing House fundraising event.
He's excited about singing alongside an assortment of other talented Kiwi singers, including fellow tenor Ben Makisi, who's been a good mate for 20 years.
But soon after that it's back on the road.
Next year is the bicentenary of Wagner's birth, so Wagner specialist O'Neill has a raft of international events lined up.
Tickets to the Hearing House fundraiser cost $225. To purchase, call 0508 266 237. Booking fees may apply.
