It was the first time in 40 years that more than eight athletes had contested the senior men's event at an Athletics Otago meeting at the Caledonian Ground.
Under athletics rules, only eight competitors can contest the final three throws in a throwing event.
O'Shea was called for fouls on his first three throws and was eliminated from the competition.
O'Shea (21) is the New Zealand 400m hurdles champion and won a gold medal in the event at the Oceania championships in Cairns, Australia last month.
Malcolm Giles, the throws referee at the Caledonian Ground, has been officiating in the throwing events for the past 18 years and said it was the first time the cut had been used at an Otago event in his time.
Longer-serving athletics followers felt the cut was last used about 40 years ago in a senior men's throwing event.
The discus has become the popular throws event at the Caledonian Ground this season.
Twenty-five athletes competed in various grades on Saturday.
Two more athletes might have been eliminated if Taieri's Marshall Hall and Kieran Fowler had been competing.
They won the gold and silver medals in the senior men's event at last years national championships in Auckland.
Matt Dallow (38), who represented New Zealand in the decathlon in 1993, comfortably won the senior men's event with a throw of 47.45m.
His throwing sequence was foul, 47.45m, foul, foul, foul, 45.80m.
Hamish Clapp was second with 41.98m and Kieran Moffit third with a personal best 36.93m.
Other top performances in the discus came from Hamish Finnie (Hill City), who won the men's aged 17 to 19 event with 41.08m, Nicole Bradley (Caversham), who won the junior women's throw with 35.42m, and Matt Aitken (Hill City), who won the AWD event with 23.50m.
Bradley also won the junior women's hammer throw with 37.43m.
Dean Rusbatch (Taieri) won the boys aged 15 and 16 hammer throw with 40.72m.
A high standard was set in the men's high jump, with Elton Matternich (Invercargill) winning the boys aged 15 and 16 event with a personal best 1.90m, from Brent Cheshire (Ariki), who jumped 1.85m.
Former international Chris Donaldson demonstrated his class by easily winning the men's handicap 60m sprint in 7.24sec.
Lauren Wilson (Hill City) won the women's sprint double against club-mate Fiona Hely.
She won the 100m in 12.53sec while Hely ran 13.15sec, and the 200m in 25.38sec with Hely following in 26.67sec.
Mrytle Rough (Hill City) broke her fifth New Zealand masters record when she won the veteran women's over age 70 3000m in 14min 08.98sec.
It beat the 1996 record run by Edith Mercier (Leith) by a massive 1min 50sec.
Alex Gorrie (Hill City) applied pressure over the last two laps to drop Glen Ballam (Invercargill) and win the men's 3000m in 8min 48.02sec.
Ballam clocked 8min 57.33sec.
Daniel Balchin (Caversham) won the men's 800m in 1min 58.66sec from Ben Jowsey (Hill City), 1min 58.93sec.
