Oh interj. : An exclamation expressing various emotions, according to the tone and manner, especially surprise, pain, sorrow, anxiety, or a wish. See the Note under O.

Example:
How does the Oh group get affected in the following reaction?

Taxonomy
D'oh, Annoyed Grunt, The Simpsons, the Oxford English Dictionary, catchphrases, free encyclopedia, search, Homer Simpson, BBC News, 09-07, Dan Castellaneta, characters, better suit, timing, animation, apostrophe, 20th Century Fox, Bart of Darkness.

Defination from the web:
... Matt Groening felt that it would better suit the timing of animation if it were spoken faster, so Castellaneta then shortened it to a quickly uttered "D'oh! ...