An atom of oxygen has six valence electrons. in nature, oxygen is a diatomic molecule and is usually found in the form o2. why would one atom of oxygen want to bond with another one?

In order to attain stability

Explanation:

Every chemical bonds that are formed results from the desires of atoms to achieve stability like the inert and nonreactive noble gases. By so doing, their outer shell electrons must be completely filled to attain an octet configuration like the noble gases with eight outermost electrons.

An atom of oxygen has six outer electrons. To complete its octet, it will require just two more electrons.  

This desire will make it bond with another atom. In the vicinity of another oxygen atom, it will bond. The type of bond formed will be a covalent bond which results from each of the oxygen atom sharing two of their valence electrons.

The joint shared electrons will now be four electrons. This four electrons will make each of their octet complete and a covalent bond occurs.