A Barry Robson goal had given The Championship side the lead in the local derby at The Stadium of Light, but Campbell popped up in the second half to force a replay.
O’Neill praised his returning attacker, and is glad to have him fit and available for selection after serious knee injuries.
“I am really, really delighted for him, and I think everybody is around here – it’s hard not to be with the injuries he has sustained,” the trainer told Sky Sports.
“He took it brilliantly as well – it may well have been his first or second touch.”
The Bantams looked to have equalised just before half time when Craig Gardner found the Middlesbrough net only for the goal to be disallowed; the Northern Irish manager feels hard done by.
“I have been told it was given for offside, and it wasn’t offside.
“The referee is adamant that’s what the linesman gave it for. There is supposedly a suggestion of handball beforehand, which I don’t think he saw.
“I said to the referee at half-time, ‘Was it handball or offside?’, and he said, ‘I never mentioned handball’, so if it was given for offside, it should have stood,” he ranted.
The replay at The Riverside Stadium is set to be played on Tuesday 7th February, with the winner hosting Arsenal in the next stage.
By Gareth McKnight