Course signage had blown over near the start, resulting in several of the early front-runners missing a crucial turn.
"It probably added 400m," O'Sullivan said at the finish.
"I suppose it only really affected my time."
But for Chisnall it signalled the beginning of a long hard slog, resulting in her finishing a little the worse for wear in 1hr 27min 42sec.
The threatening rain got heavier and the wind chill intensified, causing her some distress with breathing.
"It just became a battle for survival," she said.
"I just abandoned everything and focused on finishing."
The day did not get any better for Chisnall when race rules - she was not wearing her club vest - denied her the opportunity to claim the Otago senior women's half marathon title, although her result as Hill Free Half Marathon champion will stand.
The Otago senior women's title went to Sarah Pearson (Ariki) who completed the course in 1hr 46min 24sec.
But there was no disputing O'Sullivan's result as he claimed dual honours.
O'Sullivan (23), the defending course champion, was initially involved in a three-way battle up front with Neale McLanaghan (Leith) and Kieran Cody (Otago University).
McLanaghan fell off the pace after 5km leaving training partners O'Sullivan and Cody to battle it out.
O'Sullivan became aware of the threat Cody posed, and decided to make a break at the 15km mark.
"I don't have a great sprint, and didn't want to leave it to the last couple of ks," he said.
O'Sullivan crossed to finish in 1hr 13min 41sec, with Cody second in 1hr 15min 40sec and McLanaghan third in 1hr 18min 40sec.
Despite prolific success as a junior, it is O'Sullivan's first Athletics Otago senior title.
The associated 10km events witnessed the emerging talent among junior middle-distance runners on the Otago scene with victories to Alex Gorrie (Hill City in the open men's category and Hannah Newbould in the open women's.
Newbould (19) burst on the local scene back in May when she set a 1500m open women's record in the Lovelock Relays.
She has since broken the national junior women's 19 record, recording 4min 14sec in the 1500m at the world junior track and field championships in July.
"I needed a blow out," Newbould said of her first 10km race yesterday.
She is conditioning and preparing for a late start to the track and field season with a goal to target the national track and field championships in March.
She completed the 10km course yesterday in 36min 23sec.
It was also the first 10km race for Gorrie, whose main objective was "keep a steady pace and not blow out".
The national secondary schools 6km champion used the race as part of his preparation towards defending the title in December, displaying every sign he is more than up to competing over the 10km distance.
"I just picked it up with 5km to go, and kept getting quicker and quicker."
Gorrie completed the course in 33min 14sec, from Will Smith, second in 33min 30sec and Tom Hunt, third in 33min 38sec.
