How is -OH formed and what happens to the missing oxygen valence ...

[clippers!]

Oxygen has 6 valence electrons, leaving 2 that are free to bind covalently with another atom.  Hydrogen has 1 valence electron.  The 1 hydrogen electron can combine with one of the 2 free oxygen electrons.

This now leaves 1 free oxygen electron.  But at this point, the octet rule is not fulfilled.  What happens to that missing spot for the 8th and final electron in oxygen's valence shell?  Doesn't it have to bind with something?
Paul, you are starting with the hydroxide ion already formed.  But I am asking how does it get formed in the first place by a pairing of O and H?

[camila]

The key here is that we are working with the hydroxide anion. Because of hydroxide's -1 charge, it bears an extra electron in its structure; it is this extra electron that pairs with the other free oxygen electron to occupy that final valence slot.