The 24-year-old returned from 17 months of misery after two cruciate ligament injuries to claim a second-half equaliser which denied derby rivals Middlesbrough a giant-killing victory at the Stadium of Light.
Campbell, who was introduced as a half-time substitute, expertly converted James McClean's 59th-minute cross to announce his re-emergence in style.
O'Neill said: "Dave Galley, our physiotherapist, tells me - and Dave has been in the business a fairly lengthy time - that he may well be the best of the lot in terms of never being down.
"After the injury, to get the same injury again, you might have thought that might be it.
"But he knuckled down again, always with a smile on his face.
"He gives everybody a lift around the football club because of his attitude, almost epitomised in the way everything happened for him in that moment.
"It was great for him to celebrate. I am delighted for him."
But for Campbell's contribution, the Black Cats might have been heading out of the competition after Boro produced a performance of real character at the Stadium of Light.
Midfielder Barry Robson volleyed them ahead with 16 minutes gone, and although the Black Cats controversially had a 39th-minute Craig Gardner strike ruled out for offside, were good value for their half-time lead.
Indeed, they might have been further ahead if January signing Lukas Jutkiewicz had made the most of a glorious late opportunity.
However, Sunderland responded after the break and finally got their reward through Campbell.
Boro boss Tony Mowbray was delighted with his side's performance.
He said: "We were pleased with their effort, of course, but the dressing room is slightly disappointed we didn't come through the tie with a victory.
"They showed great spirit. Despite the nature of Sunderland's equaliser - we handed it on a plate to them, really, and we could have lost a bit of heart from that.
"But we pushed on and if anything, looked as if we were going to maybe get a winner."
The winners of the replay will be rewarded with a home tie against Arsenal, although O'Neill for one is taking nothing for granted.
He said: "It's very, very tough - and we are nowhere near there at the moment, nowhere near it.
"We have got a pretty tough game coming up against Middlesbrough."