O'Sullivan (22), a bachelor of commerce student, won the 10km section last year at the annual event held from the West Taieri Rugby Club grounds, and targeted the half marathon this year as a long steady run in preparation for next weekend's national road relay in Christchurch.
Victory yesterday also capped off a winter harrier season for O'Sullivan during which he became Otago's most improved middle distance athlete.
Last weekend, he won fastest time honours of 30min 39sec in the Port Chalmers to Dunedin Road Race, while mid-season he improved from fourth in the Edmond Cup Steeplechase to finish runner-up in the Otago cross-country championship.
Although involved with the leading bunch from the start, his tactic of tackling the first 14km at a steady pace paid off, as he was able to increase his pace over the final 7km with ease, to finish in a time of 1hr 16min 23sec, well clear of Louis Tomlin (1hr 18min 5sec) and Otago marathon champion Tom Hunt (1hr 19min 9sec).
First in the women's section of the half marathon was Hannah Musgrave first in 1hr 35min 39sec, Mel Aitken was second in 1hr 36min 44sec and Andrea McCheyne was third.
Musgrave (22) is a past age-group representative in English county netball, representing Greater Manchester.
Dougal Thorburn made short work of claiming the associated 10km open men's trophy, in 32min 28sec, with Jason Palmer second in 35min 3sec and Murray Dempster third in 38min 56sec.
Although admitting feeling sluggish, Thorburn could be forgiven, given the hail and sleet that swept the course on occasion.
Thorburn (28) has represented New Zealand in mountain running in recent months.
He finished 73rd at the world championships in Italy and went on to improve on his world performance to finish eighth in the inaugural Commonwealth Mountain Running championships, and with it a silver medal in the team section.
Like her male counterpart, Otago road champion Rachel Kingsford easily won the open women's title in the 10km event, completing the course in 36min 11sec.
It has been a golden year for Kingsford (21), a second year physical education student, with a national title in the 5000m track and field championships, backed up with a successful cross-country season where she won the North Island title, and finished fourth in the national event in August.
It was a true family affair in the associated future champions 2km race for children 12 and under, where the Sinclair family made it a clean sweep.
Hamish (11) won the title in 8min 10sec, holding out younger brother William (9) who finished second in 8min 38sec, and for good measure sister Sophie (8) finished third overall to claim the girls' title in a respectable 9min 11sec.
