The record holder is his clubmate and training partner, Andrew Whyte, who is still nursing niggly injuries and did not start.
O'Shea (Hill City-University) ran too slowly in the first 200m and his lack of pace judgement proved costly down the front straight. He was timed at 48.10sec. Stephen Isaac (Hill City-University) was second in 50.19sec.
The 47.35sec O'Shea ran earlier in the month remains the best time in the country this season.
His time was not fast but Ben Anderson's ferocious last lap in 55sec suggests that he could be a dark horse at the New Zealand 1500m championships.
Nathan Baxter (Ariki) was the pace maker and led until the final bend when Anderson took off and won by 20m in 4min 0.24sec. Baxter was timed at 4min 2.09sec.
Rozie Robinson (Hill City-University) is on target to win her third successive New Zealand race-walking double in Auckland next month.
She proved it on Saturday when she dominated the Otago 3km race walk championship and won in a season's best time of 14min 16.91sec.
She beat Lesley Cantwell (Te Anau), who was runner-up to her in the national championships last year, by 23sec. Cantwell (26), who is ranked second in the country, broke the Southland record with her time of 14min 39.97sec.
Julie Edmunds (Caversham), who is ranked fifth in the country, was third in a new Otago masters aged 40 to 44 record time of 16min 49.23sec.
Robinson (22) has won 10 New Zealand titles and is poised to win two more in Auckland in the 3km track walk and the 20km on the road.
Her target is to qualify for this year's World University Games in Russia in the 20km walk. To do that, she must reduce her best time from 1hr 45min 49sec to 1hr 39min.
Rory McSweeney (Taieri), who last week qualified for the world Paralympics in Lyon, France, went even better when he won the senior men's javelin title with 49.24m. It was a personal best by 1.65m.
McSweeney, who competes in the CF 44 below-the-knee amputee grade in Paralympic competition, beat the 45m standard for the world championships with five of his six throws.
His sequence was 43.91m, 47.01m, 46.69m, 46.33m, 49.24m and 48.37m.
Tori Peters (Gore), the top-ranked senior woman in the country this season with 46.80m, won the junior women's javelin with 44.73m.
Greer Alsop (Southland), who is ranked second in the country this season, won the women's triple jump with 12.38m.
Fellow Southlander Todd Swanson, who is fourth-ranked in the country, won the senior men's triple jump title with 14.32m.
Jerram Huston (Taieri) won the senior men's shot put with 14.62m from clubmate Dean Rusbatch 13.82, who equalled the Otago men's 19 record for the 7.26kg shot.
Maddy Spence (Hill City-University) turned the tables on New Zealand secondary schools champion Christina Ashton (Taieri) when she won the women's aged 16 and 17 100m hurdles in 15.29sec. Ashton was timed at 15.54sec.
Ashton (15) won the 100m in the same grade in 12.80sec from newcomer Caitlyn George (Hill City-University) 12.81sec.
The senior women's title was won by promising North Otago sprinter Jasmine Ng (15) in 12.80sec. Ng also won the girls 15 and under 200m in 26.30sec.
The fastest male sprinter was Blair Grant (Taieri), who won the senior men's 100m in 11.28sec and the junior men's 200m in 23.70sec.
Oli Chignall (Hill City-University) ran the fastest 3000m of the day when he won the boys 15 and under title in 9min34.13sec. It was a personal best time by 9sec.
Shaun Markham (Hill City-University) won the junior men's AWD shot put with 2.50m and club-mate Matthew Aitken the senior men's shot put with 7.41m. Nikita White (Hill City-University) won the AWD junior women's 200m in 32.97sec.
Liz Wilson (Hill City-University) broke the Otago masters women's aged 50 to 54 record when she won the masters women 200m in 27.42sec. Claire Giles (Caversham) was second in 30.84sec and set a masters aged 55 to 59 record.
Myrtle Rough (Hill City-University) set an Otago masters women's aged 70 to 74 800m record of 3min 18.98sec.
