2. a response to a suprising situation/coment/picture.
3. a conversation scapegoat.
4. a sarcastic reponse.
5. see oh dear
6. see jonathan costa
7. an old person term.
It is most commonly used as a simple image macro featuring George Takei; the meme itself probably originated from instances of Takei saying "Oh, my!" in response to sexy stuff, especially on the Howard Stern show.
Jonah Falcon: K. *whips out 13-incher.*
George Takei: Oh my! :]