“Ooh, my leg's asleep.”
You've been watching a movie while laying on your sofa in a strange position. Now your leg feels strange and it's hard to move it. You say this to your girlfriend.
Ooh, my leg's asleep.
When part of your body like your arm, leg, hand, etc. is in a strange position for a while, it starts to feel tingly. It feels like someone is poking it with lots of little needles. You describe this situation by saying that the body part "is asleep":
My foot's asleep!
You can also say that the body part "went to sleep":
My arm's gone to sleep. Can you move over for a minute?
People say "Ooh" when they're surprised by something, or when they suddenly notice something.
Ooh, I forgot to tell you: I'm going out of town next weekend.
Ooh, my back is sore!
You can use "Ooh!" for either positive or negative things. But there's another version of "Ooh" that's pronounced with a longer sound which has a completely positive meaning: