Since 1987, O'Hagan was involved as an athlete, coach of O'Hagan Sport and administrator in triathlon.
Cameron Brown, New Zealand Ironman champion and close friend, remembers the first time he met O'Hagan at the start line of a race in Tauranga in 1989.
"I beat him by five seconds. That was my first memory of Tony and there were many more close finishes after that."
Brown had just come back from a five-hour bike ride when the Herald called and said he'd been thinking about O'Hagan the whole time.
"I have fond memories from when I was a 17-year-old racing him to when he was an Ironman athlete, he was one of those guys that was just such a tough bugger.
"He would put 100 per cent into every race. I was always worried when Tony O'Hagan was at the start line.
"He will be very sadly missed."
World Sport Coach wrote on Facebook: "An incredible person, friend and great coach who was totally dedicated to supporting his athletes and always willing to learn and try new things.
"RIP Tony, thank you for your incredible contribution to the human race and endurance sports."