Answer:
  This question seems to have profound meaning. Anyway, I will try to answer as much as I can.
 
  First of all, the O-H bond in the hydroxide ion, as well as in the water molecule, is a covalent bond. 
 
  Let me give you the example of a water molecule (H₂O). The O-H bond in H₂O is a covalent bond. Both the hydrogen atom (H) and the oxygen atom (O) donate one electron to one another in the O-H bond, so that both atoms share two electrons. Since the oxygen atom pulls the electron more strongly than the hydrogen atom (electronegativity of the oxygen atom is higher than that of the hydrogen atom), however, the shared electrons in the O-H bond are slightly shifted to the oxygen atom.
 
  Suppose that a proton (H⁺) is eliminated from a water molecule, namely,
 
  H₂O « OH^- + H⁺ 
 
  Because the electron which the hydrogen atom shared in H₂O is now left behind with oxygen atom, the eliminated proton does not have any electron. The O-H bond in the hydroxide ion is basically same as that in the water molecule, that is, it is still a covalent bond.
 
  But once we start thinking “where in the oxygen atom is the extra electron located?”, it becomes a more difficult question to answer.
 
  Many current textbooks seem to teach that the structure of the water molecule is an sp³ hybrid orbital. However, another theory based on spectroscopy and molecular orbital method, indicates that water may have an orbital similar to the oxygen atom’s rather than an sp³ hybrid orbital, is also influential in current structural organic chemistry. Therefore, I am afraid that I cannot clearly say where the extra electron is located (which orbital it belongs to). 
 
  Let’s take a look at the example of methane (See picture at left, from Dr. Philip J. Grandinetti, with permission). When a proton is eliminated from methane, the reaction formula is same as in the case of water;
 
  CH₄ « CH₃^- + H⁺
 
  In this reaction, the carbon atom takes an electron out of the eliminated hydrogen atom. The carbon atom now has 7 electrons, which is one more than the number of electrons the carbon atom usually has (6 electrons). So the charge “–1” is added to the carbon atom.
  
   It is well-known that the negatively charged carbon atom, as well as methane, forms sp³ hybrid orbitals. 4 orbitals are spread out from the carbon to each apex of pyramid. 3 out of 4 orbitals are used for forming C-H bonds, and the remaining orbital has an unshared electron pair. If the oxygen atom in the water molecule also has sp³ hybrid orbitals, the property would be similar to that of methane; however, it is actually very doubtful.
 
 
Acknowledgement
  We would like to thank an anonymous professor for his careful teaching and kind support for this answer.
 
 

This article is translated by Chemistryquestion.com from the original article in Chemistryquestion.jp.  Please let us know if you find any errors.