Strange valence numbers? I'm trying to understand why ChemReference sometimes lists the valence electrons as a number I don't expect. Take Oxygen, for instance. It should have six valence electrons, however the site lists two. Why?

I hear that, for transition metals, the d-orbitals count towards the valence. Is there another rule like that I need to know about that might explain Oxygen?
Look closely on that site. It's not 'valence electrons' but 'valence'. There is a difference between the valence electrons and valence. 

Valence electrons are the number of free electrons in an atom that can take part in making the bond. There are 6 such electrons in the oxygen atom as you stated.

While the valence or valency is the number of bonds that can be made by the atom.

That's why valency of oxygen is 2