People say "Ooh" when they're surprised by something, or when they suddenly notice something.

For example:

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:

Ooh, those make you look so intellectual!