O'Shea (24), an accountancy student at the University of Otago, won the 23rd annual Time Buster in 34.31sec and was followed home by clubmates George Woodhouse (35.96sec) and Sam Bremer (37.07sec).
The 243m street race starts and finishes outside the New Zealand Shop when the town hall clock strikes the first of its 10 strokes at 10am.
The aim is to reach the finish before the clock strikes 10. O'Shea was just 5m short of the target when the clock struck 10 and failed to add the bonus of $250 to his winner's purse of $300.
In the history of the Dunedin street race, only three runners have beaten the chimes.
New Zealand 400m champion Cory Innes (Taieri) was the first when he ran the record time of 31.63sec in 2008. The following year, winner Andrew Moore (Hill City) and clubmate Toby Flett both beat the chimes but not the record.
O'Shea was disappointed in missing out on his bonus payment.
''But I was happy to get through the race unscathed,'' he said.
''My main goal comes later in the season and it was important to look after my body on the downhill.''
O'Shea, who beat a 41-year-old Otago senior men's 400m hurdles record last season, is on target to make the New Zealand team for the Commonwealth Games in Glasgow in the event.
O'Shea hugged the inside track and ran a controlled race downhill to the Regent Theatre before tackling the difficult uphill stretch back to the New Zealand Shop.
''I went 100% up that last hill and tried to bring it home fast and beat the chimes. It was just like the end of a 400m race on the track.''
He intends coming back next year in an attempt to beat the chimes and claim the record.
Woodhouse (17) was second behind O'Shea going past the Robbie Burns statue and held on to the place in only his second attempt at the race.
He finished sixth in 2011 and used that experience to open up a gap on the rest of the field on the downhill stretch.
Taieri's Zoey Flockton (18) worked hard up the first hill to the Robbie Burns statue and held on to win the women's race in 40.03sec from veteran Liz Wilson (Hill City-University) 40.81sec and Anna Cross (Hill City-University) 42.05sec.
Flockton, a physical education student at the University of Otago, was competing in her third event and was runner-up last year.
''I knew what I had to do and got to the front and held the inside down the hill,'' she said.
''The hardest part was going back up the hill to the finish. I could hear the runners behind me getting close.''
Wilson (51), the world masters 400m champion in Brazil last year, was competing in her fourth Time Buster.
Her first was in 2010, when her daughter, Lauren, won the women's race. This year, her son, Aston, was sixth in the men's race. On both occasions her children ran faster times.
