The Hoops defeated Dunfermline Athletic FC 3-1 in Saturday thanks to goals from Daizen Maeda, Arne Engels, and Kelechi Iheanacho.
Reflecting on the achievement, O’Neill praised his players and admitted he never imagined returning to deliver such success again at this stage of his career.
“I feel ecstatic. I never thought that I’d manage Celtic again, but to end up winning the double is just the best feeling in the world," he told the media.
“It was lovely to get on the field there and see the Celtic fans, and us winning the game, which was a culmination of the second part of the season, and we ended up winning the league a week ago, and you just couldn’t make it up.
“I thought we had a lot of possession in the first half, Daizen scored again, and I thought we should have pushed on more strongly when we got the first goal.
“We didn’t and we took the sting out of the game, which was absolutely fine. Then we got the second goal and you feel you’ve got a bit of a cushion."