Benitez, whose side visit the Stadium of Light today, has not been helped by failing to win any of his first three Premier League matches.
O'Neill said: "Roberto Di Matteo, having done what he did there, was held in very high esteem by the Chelsea faithful.
"There has to be a crowd reaction.
"Nothing surprises any of us too much about football but I was genuinely taken aback by him being ousted. Ask other managers and I think they'd say the same.
"You'd have to say it was a major surprise that Di Matteo went after everything he'd done for Chelsea in recent times.
"They are the FA Cup holders and won the Champions League.
"That was the holy grail for Chelsea and they won it."