Thierry Henry scored the winner in his final Premier League game for Arsenal before he returns to New York, as the Gunners beat Sunderland 2-1 at the Stadium of Light.
Jamie McClean had Sunderland's first real chance of the game but headed his attempt over, while Arsenal's two best chances of the first half fell to Theo Walcott.
First, the England winger fired across goal from a Robin van Persie cross, before he volleyed harmlessly over from an Alex Song centre.
Van Persie, scorer of 28 Premier League goals, was well marshalled by Michael Turner.
The game erupted into controversy six minutes before the break when O'Neill's men thought they should have been awarded a penalty.
Mertesacker controlled Jack Colback's cross on his chest, but then saw the ball rear up off the turf and hit his arm.
Referee Neil Swarbrick was having none of it.
With both teams cancelling each other out in the second half, Arsene Wenger replaced Alex Oxlade Chamberlain with Thierry Henry. However, it was Sunderland that struck first.
Mertesacker appeared to catch his studs in the turf and, as he collapsed, McClean seized on the ball to score his second game in two matches.
The German defender had to be stretchered off to be replaced by Aaron Ramsey, who took just three minutes to equalise. The ball broke to the Welshman 20 yards out and his shot hit both posts before going in past Simon Mignolet.
The last word was left to Henry, as he collected a fine Andrey Arshavin cross before turning the ball past Mignolet.