Mr O'Leary knows that getting on to the big stage is not easy and he still has a lot of hard work ahead of him, despite some unusual training already.
The games are being played throughout a big country where the temperatures at kick-off time range from 15C to 35C. With Mr O'Leary covering 12-15 kilometres during a game he prepared by doing lengthy runs around Whangarei wearing a woolly hat and long jumper "to stress-test my body in the heat".
"Unfortunately we don't have a humidity chamber so I've been trying to challenge myself like that," he said.
He leaves Whangarei today and will arrive in Rio tomorrow before going to work on Monday, which will involve extensive training, fitness testing, seminars and refreshers on the rules of the game and a lot of video analysis of his and other referees' performances.
"We do a lot of video analysis of our performances. Just like the players, we are constantly looking at how we can improve our performance."
His reffing style involves letting the players play football, keeping the game flowing and only intervening if they "stop playing the game".
"The players are the stars and who everybody wants to see, not me," he said.
Mr O'Leary admits, though, that he's starting to get a bit excited, and a tad nervous, about the prospect of performing on the world's biggest stage. But what do Tikipunga High students think of him possibly rubbing shoulders with some of the biggest names on the planet?
"Some of the girls want me to get [Real Madrid and Portugal star] Ronaldo's autograph, especially after he ripped off his shirt in the [European] Champions League Final last week. The boys in the school seem to be pretty excited about it. It gives them a great connection to the tournament, it makes it within touching distance."
The World Cup kicks off on June 12 with a match between hosts Brazil and Croatia, and referees find out if they will officiate in a game only two days beforehand.