Mr O'Sullivan, a sous chef, snow sports instructor and Wanaka Volunteer Fire Brigade member, recently became an unwitting individual entrant in the event, which takes place on January 21.
"It turned out the decision was made for me in October to do it [alone]." "Originally the idea was to stick a fire service team in there. But getting a whole team together was a bit tough with the training commitments, so it more or less ended up just being me."
While he completed Challenge Wanaka's marathon leg as part of a team last year with just "a couple of weeks' notice", taking on the entire event was a whole new ball game, he said.
"When you start thinking about the sheer distances it's distinctly daunting."
His main motivation is to raise money for the local St John Youth organisation.
"For a lot of those kids it gets them back into some competitions which I'm all for and it's something which directly benefits the community."
He also wants his solo effort to encourage more Upper Clutha emergency service volunteers to enter future sporting events in the area.
"We see each other throughout the year quite a bit and often it's not at the most pleasant places so if you can go and do something in a nice environment with each other and get to know each other, it makes life a lot easier."
Despite being a triathlon novice, Mr O'Sullivan is no stranger to sporting contests. As a former speed skier, he competed at World Cup events, travelling at speeds of up to 160kmh, and earning four top 10 finishes before retiring from the sport in 2010.
He was also involved in competitive road cycling when he was younger, including a stint in Europe.
Improving his swimming technique for the event - "good lord was it ugly" - has been the biggest training obstacle.
"I've got a bit of a speed addiction. Going fast on skis, going fast on a bike. So doing something slow like swimming - I'm just not interested."
Mr O'Sullivan said he was copping plenty of "good-natured ribbing" from his fellow firefighters, which would probably continue on race day.
"I don't know if they'll cheer me on or sit round having a beer and having a good laugh."
He insisted his attempt at the full Challenge Wanaka distance was strictly a one-off.
"This is me done. Unless of course I don't finish and I'm going to be back next year, which I really don't want to happen.
"I'm either going to miss the cut-off time or the ambos are going to drag me off the course. There's going to be no quitting."
