O'Neill said: "Losing the title on goal difference is murderous. It is terrible — the worst. All the missed chances from the whole season go through your head.
"It was particularly hard on the players because we were part-time and Colchester were full-time.
"Sir Alex Ferguson will be the most disappointed man in Britain if they lose on goal difference.
"We are bit-part players here at Sunderland, but we do have our own ambitions. We could still finish in the top 10."