"I'm just really happy to come away with the win. The 3.3km distance seems to suit me all right; you can put a bit of pace on at the start which is good," said O'Brien, who won in a time of 37 minutes 12 seconds.
Radford's mistake cost him second place as Cook Strait record holder Casey Glover passed him during the mix-up, with the national champion settling for third.
As expected, Australian Melissa Gorman destroyed the rest of the field in the women's race to win in 39:10, which was fourth place overall.
Gorman, the 2009 world champion, was nearly three minutes ahead of second place-getter Emma Robinson from Wellington's Capital club.
"I really wanted to see how long I could stay with the boys. That was my main priority going into this race," said Gorman.
This victory makes it two wins from two attempts in New Zealand for the 26-year-old after she won round two of the State Ocean Swim Series in the Bay of Islands last December, where she beat all the men home. Having already qualified for the Olympic Games, Gorman said she is using the series to prepare herself for London.
"I wanted to get a good hit out. I don't get too many race opportunities in the lead-up the Olympics ..." she said. "I think I'll be back to New Zealand for one more of the swims and I have a couple of events coming up at home in Australia so I've got quite a busy few months, then a hard training block leading into London."
The State Capital Classic boasted the participation of many elite athletes, but it also accommodated almost 1000 swimmers from children to retirees who competed in one of four different distance swims on offer.