"I don't mind if it's only one game... especially if that game's the final," O'Leary said.
"Really though, it's an honour and a privilege to be selected to officiate at the World Cup. In effect I've made the first team squad and now I've got to prove that I'm good enough to make the first XI."
He's already undergone plenty of testing and there's more ahead of the cup, with a trip to Fifa House, in Zurich, for a final workout before the big event. The final testing covers everything from physical and psychological testing to knowledge of laws of the game to drug testing.
"Yes, we also get drug tested, just like the players, and we've got a list of all the banned substances. Thankfully caffeine from coffee isn't on there or I'd be no good."
O'Leary describes his style of refereeing as letting the play flow with as little intervention as possible, though he's not afraid to use his cards if needed.
"I like to let the players dictate how they are going to play and referee accordingly," O'Leary said.
So what's it going to be like in the red hot atmosphere with 100,000 fans in the stadium and hundreds of millions watching on TV around the globe?
"For me, I'll be happy if nobody talks about me as a referee as that would mean I've done my job well. The referees often get talked about if they have made a mistake - and everybody makes mistakes - but I always want to be the invisible man."