The Black Cats stunned the Blues last season when they returned from Stamford Bridge with a 3-0 victory.
This time around, they head for the capital on the back of a promising revival since O'Neill replaced Steve Bruce as manager in December, and having taken 13 points from a possible 18 and secured their place in the FA Cup fourth round.
That run has eased the Wearsiders into the top half of the table, although the Northern Irishman is adamant that he will continue to look over his shoulder.
He said: "Of course I am concerned, yes. If this was the final league table of the season, that becomes a different issue, but it is not.
"We could lose tomorrow and we could drop six places in the league, so it's still very, very tight.
"What we have been able to do, we have been able to haul ourselves into a pack of teams that I am quite convinced wouldn't be looking in any other direction than over their shoulder.
"Chelsea will be a difficult game for us, exceptionally difficult. But we are going in with confidence and we will see.
"If we win the game, it's three extra points for us and if we do that there, that would be fantastic for us, and that's the way I would look at it.
"Similarly, if we are beaten in the game, I'm hoping that it wouldn't dent confidence so badly that we would fall apart."
O'Neill's stance on the club's position has not wavered since he walked into the Stadium of Light, although asked if he is a man who generally views the glass as half-full or half-empty, he said with a smile: "It depends. It depends if it's half-full."
While Sunderland's recent form has edged them out of trouble, Chelsea have rather fallen off the pace in the race for the league title to leave manager Andre Villas-Boas and his players under pressure.
However, O'Neill is not being fooled by talk of their demise.
He said: "To start suggesting that these players are over the hill and not capable of contesting big Premier League games is just utter nonsense.
"They are still in the Champions League and they are sitting fourth in the league.
"They might be disappointed themselves that they have dropped more points at this stage than perhaps they thought they might do, but they are still a very fine side."
O'Neill could be boosted by the return of striker Nicklas Bendtner from a knee injury, while fellow frontman Fraizer Campbell was among the travelling party which left this afternoon for the first time since suffering a second serious knee injury in August 2010.
The manager said: "It is nice, but he has been out a fairly lengthy time, so we don't expect miracles at this moment. Just to get him fit and stay fit would be good news for us, and anything he does after that would be wonderful."
Midfielder Craig Gardner will also be at O'Neill's disposal with his manager desperate to hang on to him amid speculation that homesickness could force him back to the Midlands.
He said: "Obviously, we would like him to stay, but we will see what develops and at this minute, nothing has developed."
Little has developed either on the incoming front with O'Neill adamant he is yet to sit down and discuss targets, although he intends to do that within the next few days.
He said: "I genuinely haven't had that much time to think about it. Of course, it crosses your mind, but to sit down and start going through things, I haven't had the chance to do that.
"I will do, obviously, next week."
