Goals from Yang Hyun-jun and Reo Hatate put Celtic in front after Ayase Ueda had given Feyenoord the lead, before Benjamin Nygren’s late goal sealed a landmark victory.
It is Celtic’s first win in the Netherlands since 2001, when O’Neill was first in charge and they knocked Ajax out of Champions League qualifying.
“We played really well,” he said. “There was a period where we were dominant, controlling the ball and exuding a lot of confidence, which was lovely to see.
“Going a goal behind, we could have crumbled. But we showed character and resilience and then when we got the equaliser, the confidence flooded into the side.
“I said to the players ‘you will get a chance to play tonight’. We took that chance.”
Celtic play Roma and Utrecht at home, either side of a trip to Bologna, in their three remaining league-stage fixtures. They sit 21st in the 36-team table on seven points.
“Judging from last season, you are looking for 10 or 11 points to qualify,” O’Neill added. “It won’t be easy, but Celtic have two home games and the confidence is in the side now.”
Columbus Crew head coach Wilfried Nancy appears close to being appointed as Rodgers’ permanent successor and O’Neill thinks Celtic are well placed for the new man in charge, whoever it may be.
“He has got some players who are big winners,” O’Neill said. “I’m sure he will lean on some of those lads and then it’s about improving some of the other players.
“The restoration of confidence is big and it’s keeping it going after that.
“Winning away from home is terrific – it’s not easy away from home in Europe. It gives them belief that they can come and compete.”