Olympic gold went to Great Britian, silver to Australia and bronze to France - all three qualified from the Irish semi-final.
Indeed in the heats O'Neill and colleagues finished ahead of the French crew.
In their B final the Irish crew of O'Neill in the stroke seat, Sean Casey, Jonno Devlin and bowman Cormac Folan hit the finish line in six minutes 7.97 seconds.
In warm temperatures and slight tail wind conditions it was enough to see the St. Michael's Rowing Club member and crew beat the Italian and Belarussian crews to fourth.
The Irish went out hard, covering the first 500 metres in 1:28.51 to settle in fourth place, behind Belarus, the USA and early leaders the Netherlands.
Struggling to maintain their stroke rate, O'Neill and company slipped back to sixth by the halfway point (3:03.76).
But they fought back strongly over the final 1000m, putting in split times of 1:33.56 and 1:30.65 to overhaul both Italy and Belarus.
Last Wednesday the Pallaskenry rower and colleagues finished sixth in their semi-final - a result which saw them miss out on a chance for a medal and put them into the B final. That B final was delayed by 24 hours due to the thunder and lightning.