The Hill City-University sprinter wanted a time close to his best at the Caledonian Ground but he ended 1.26sec short.
O'Shea (24) won the senior men's 400m in 48.61sec from clubmates Robert Jopp (49.67sec) and Toby Flett (51.18sec).
He was on target to break 48sec until he went head-on into the strong easterly wind in the front straight.
''The headwind was stronger than I anticipated,'' O'Shea said.
''My legs went away from underneath me.''
It was a good effort because O'Shea was still feeling the effects of mild concussion suffered when he fell down the steps at his home and banged his head last Tuesday.
O'Shea's first 400m against top opposition will be in Hamilton on December 14 against the other contenders for a place in the New Zealand 4 x 400m relay team.
The 400m hurdles is his main objective this season and he will be focusing on that when he has three Australian meetings before the New Zealand championships in Wellington at the end of March.
O'Shea is the New Zealand champion in the event and broke Roger Johnson's 40-year-old Otago senior men's record last season when he finished third at the Australian championships in Sydney in 50.16sec.
He is just 0.46sec short of the qualifying standard for the Commonwealth Games in Glasgow.
Jopp (18), a pupil at John McGlashan College, was heading for a personal best time when he hit the wall at the top of the final straight.
His time was 1.26sec short of his personal best time of 48.44sec that he ran when winning the Oceania title in Tahiti in June. He is the top-ranked junior in the country.
His immediate target is to win his specialist event at the New Zealand secondary schools championships in Hamilton next month.
Libby Jones (Taieri) won the women's aged 15 to 17 shot put with 12.41m. It was a personal best throw by 30cm.
It would have been an Otago record two months ago but the Athletics Otago records committee lifted the standard to 12.60m from 12.40m.
Jones, a pupil at St Hilda's Collegiate, won a bronze medal in the women's under-18 hammer throw at last year's national championships in Auckland.
She expects to reach the podium in three events when she competes in the senior shot put, discus, hammer throw and javelin at the New Zealand secondary schools championships in Hamilton.
Another top performance in the throws came from 16-year-old Bradley Tarleton (Taieri), who broke the 50m barrier for the first time with his winning throw of 50.03m in the men's aged 15 to 17 hammer throw.
Julie Edmunds (Caversham) broke the Otago masters aged 40 to 44 3km walk record by 10sec with her time of 16min 39.89sec.
