The full-back had the final say at Rodney Parade, calmly landing a conversion in the 80th minute to seal a 23-21 win over Lyon and spark wild celebrations among players and supporters alike.
The kick completed an astonishing turnaround, with Dragons trailing 21-6 deep into the final quarter before scoring three late tries to snatch the contest.
"It feels awesome and the most important thing was that we won," said O'Brien, who led the side in the absence of Wales lock Ben Carter.
"The boys showed great character, grit and determination. I am so pleased for everybody that we managed to get over the line.
"It's important we build on that now and take great confidence from the way that we finished the game."
The victory was Dragons’ first in 364 days, stretching back to a Challenge Cup success at Newcastle in December 2024, and their first home win in 14 months.
O’Brien had earlier put the hosts on the board with two first-half penalties, but Lyon looked to have the contest under control after tries from Janse Roux, Arthur Matron and Ave Maalo.
However, momentum shifted late on. Harri Keddie crossed twice in the closing stages, with hooker Oli Burrows also scoring as Dragons took advantage of Lyon’s yellow card.
O’Brien then stepped forward with the decisive kick.
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Cardiff were also winners in the same tournament with Callum Sheedy enjoying his match-winning moment.
Sheedy’s late penalty sealed a dramatic Challenge Cup win for Cardiff and capped a remarkable comeback against Ulster at the Arms Park.
The fly-half held his nerve to slot the final kick of the night, completing a stunning turnaround from 26-12 down and ensuring Leigh Halfpenny’s return to Cardiff ended in victory.
With the clock in the red and tension gripping the ground, Sheedy’s strike proved decisive in a contest that swung wildly from start to finish.
"It is nice with times like that when the hard work pays off," said Sheedy.
"I'm really pleased it went over. Those kicks come once a season or once every couple of seasons, so I'm chuffed."
Cardiff had made a fast start, surging into a 12-0 lead thanks to tries from lock George Nott and teenage winger Tom Bowen.
Ulster gradually wrestled control back, however, with captain Tom Stewart and centre James Hume crossing before Werner Kok struck twice to put the visitors firmly in charge with 14 minutes remaining.
With Cardiff staring at defeat, the hosts dug deep. Replacement prop Danny Southworth powered over to reignite belief before flanker Dan Thomas added another score to bring the match back to a single point. That set the stage for Sheedy, who approached the decisive kick with calm focus despite the weight of the occasion.
The 30-year-old explained how he locked into his routine in the closing seconds.
"With a match-winning kick it focuses you even more and you go back to your process.
"It was actually the stillest night I've ever had at the Arms Park. Usually the wind's going everywhere, so someone's looking down on me."
Sheedy’s kick sparked celebrations among the Cardiff players and supporters, sealing a memorable European night and underlining the fly-half’s importance in clutch moments.
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Ospreys head coach Mark Jones was quick to highlight his side’s fast start as the key factor behind their convincing Challenge Cup bonus-point win over Montauban in France.
The Swansea region made it two wins from two in the competition, backing up their narrow 24-21 success against Connacht with a far more emphatic performance against the struggling Top 14 outfit.
Five tries and a controlled display saw Ospreys take control early and never relinquish it, much to the satisfaction of their head coach.
Reflecting on the performance, Jones said: "We started well. We talked around getting the speed right, particularly against this very physical forward-orientated side.
"We knew we needed to take their legs away and the accuracy in the first 30 minutes, with the intent to play, was the deciding factor and gave us confidence."
That early dominance translated into points, with Ross Moriarty, Morgan Morse, Keelan Giles, Efan Daniel and Kieran Hardy all crossing the whitewash.
Captain Jack Walsh added eight points with the boot to ensure Ospreys claimed a second straight bonus-point victory and strengthened their position in the pool.
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Dwayne Peel admitted frustration outweighed satisfaction after Scarlets were overpowered by reigning Champions Cup holders Bordeaux-Begles, despite moments of promise in an ultimately punishing 50-21 defeat in France.
The Welsh region briefly stunned the sell-out Chaban Delmas crowd when Fletcher Anderson crossed early to give Scarlets an unexpected lead.
Any hopes of a famous upset were quickly extinguished as Bordeaux responded ruthlessly, running in five first-half tries, with fly-half Matthieu Jalibert pulling the strings and crossing twice.
Scarlets showed greater resolve after the break and enjoyed spells of pressure, with Henry Thomas scoring to keep the contest alive for a time. The hosts, however, underlined why they are defending champions by adding three more tries, stretching a tiring Scarlets defence and keeping firm control of the match.
Anderson’s second score late on reignited hopes of a consolation bonus point, but Scarlets were unable to find a decisive final breakthrough, leaving them still searching for a first win in Pool 4.
After the game, head coach Peel focused on effort mixed with missed opportunities. "It is obviously disappointing that we didn't get anything out of the game, we were fighting hard for that fourth try at the end.
"I can't fault the effort of the boys, it was just our inaccuracies that hurt us out there, you can't allow a side of the quality of Bordeaux easy ins and I felt we did that at times.
"Our forwards carried strongly, I thought Fletcher Anderson at eight was excellent, he has been a really good signing for us."
