Question

What makes hydroxide ion act as nucleophile and why does it get an overall charge of -1?

Answer 1

Nucleophiles are either fully negative ions, or else have a strongly - charge somewhere on a molecule. In the case of the hydroxide ion, there is a full negative charge on the oxygen, as well as three lone pairs of electrons.

When water ionizes one of the hydrogen atoms absconds with itself and leaves it's electron behind, giving us the hydroxide ion. The extra electron gives hydroxide a net charge of -1.