Chelsea were without the suspended Michael Essien and injury-hit Frank Lampard in midfield, an area which the Wearsiders dominated through the graft of Lee Cattermole and Jordan Henderson.
Added to the home side's woes were the absences of England defender John Terry and Alex, and Sunderland's front two of Danny Welbeck and Asamoah Gyan capitalised to great effect.
"I don't think many teams do come here, play well and get the result, because you see teams play well but in the end Chelsea just grind them down," the 24-year-old continued.
"I don't think they had conceded a goal at home here in the league until this game, so I don't think there were many people putting money on us to get a result, but we were quite confident.
"The way Chelsea were, once they were down at home, they had to press and with the two strikers we have and the form they are in they were always going to have a chance.
"The fact Chelsea had a few people missing today definitely opened the door for us."
Gyan and Welbeck netted the second and third goals, but it was the on-loan Manchester City player himself who opened the scoring with a fine solo effort moments before half-time.
"I don't watch replays of them [goals]," Onuoha added.
"Once the game's gone, the game's gone, because if you try to dwell on things you won't be able to focus on the next game.
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