Oh No! 
My first thought was "Oh crap." Then I had this flashback of elementary school with this one program we always used on the computer for fun where you did something and the computer would yell "oh no!" Every single time... XD Can't remember much more. It was like an eraser or something.
Well, with Tsuna's face and all... XD
I suppose you could use it for a "sorry..." like you forgot a birthday, anniversary, etc... Seems to fit for an apology of screwing up or something.
