​おー:

This is an interjection​ (a word used to express an emotion or sentiment on the part of the speaker).

​In this case, as you may have guessed by how it sounds (o~ is quite similar to wow), it shows amazement at something.

However, beware that it doesn't always show amazement. You've got to figure out its meaning from the context in which it was said.

​

The dash (ー) after the お character is called chouon.
It is primarily used to extend the vowel sound that comes before it.

Although it is supposed to be used with katakana only, manga directed at young children use it a lot with hiragana since it is easier to read that way.