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."
He added: "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."