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 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 (Saturday) 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."