Chemical bond-covalent-valence electrons OH-
In the photo the orange electrons are non bond electrons.
And the black electrons are bond electrons.

There are 2 electrons took part in creating chemical bond in $\ce{OH}$.
The first one is from oxygen.
The second is from hydrogen.

This kind of bond is called "Covalent bond".
For more about Covalent bond : 
https://en.wikipedia.org/wiki/Covalent_bond

The minus is  related to  the black and white electrons that are free (in Oxygen).

The white electron is come from other element.