Ulster director of rugby Les Kiss admitted his side were "total rubbish" in the first half at Oyannax before they turned the game on its head to keep their Champions Cup hopes alive.
They were 23-0 down at the break after an insipid display, but second-half tries from from Rory Scholes, Craig Gilroy and Kyle McCall allowed Paddy Jackson the chance to complete the comeback with a thumping penalty.
It was sweet relief for the province, but Kiss said there were plenty of negatives to analyse despite victory.
“It was bloody surreal – a game of two halves," he told RTÉ Sport. "We know the game is a game of two halves. They did what they had to in the second half.
"There was a bit of joy in the dressing room to get the result but in all honesty that first half was total rubbish.
"We have to ensure that doesn't happen again because a better team that can hold it together doesn’t give that back. We talked about what we were going to do in that first half.. and we were well off the mark.
"It just was not acceptable. Rory stood up and was as good a leader as you would expect at half-time and we delivered some words that were necessary. But to go out and do what we did n that second half was pretty special in a lot of ways.
"Paddy Jackson was unbelievable in that second half, he has grown so much. He’s a joy to coach. He’s going to get better and better. It was nice to have that second half, but you can take that first half and give it to someone else."
McCall thanked his team-mates for paving the way for his first Ulster try.
The Ireland U20 prop scored his side's third try to cut the gap to two points and allow Jackson to grab the glory with his 77th-minute penalty.
"We were pretty disappointed with our first-half display. We needed to front up and we didn't, but fair play to Oyonnax, they did," said McCall.
"We wanted to restart the game in the second half and really go at them. It was about getting our heads right at the scrum, working on our technique and going at them because we had to get everything right in that area.
"It was my first try for Ulster, which I am very happy with, but it was a team effort really. We really had to dig deep to overhaul Oyonnax.
"I was off the field when Paddy Jackson kicked the winning penalty and after that it was just a question of not giving away any penalties and staying strong."
The win cut the gap at the top of Pool 1 to six points and Ulster get the chance to gain revenge for their home defeat against table-toppers Saracens in the return leg at Allianz Park in round five.
"We are still in a good position and we can take confidence from our second-half performance. We can take it into next week and hopefully show what we can do for two halves," added McCall.
The defeat for Oyonnax was their fourth in a row in their first season in the Champions Cup and their eighth in their last nine matches in all tournaments.
"It was a classic game of two halves and credit to Ulster for nicking it in the end," lamented Oyonnax second row George Robson.
