Hydroxide
Hydroxide is a diatomic anion with chemical formula OH. It consists of an oxygen and hydrogen atom held together by a covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. It functions as a base, a ligand, a nucleophile, and a catalyst. The hydroxide ion forms salts, some of which dissociate in aqueous solution, liberating solvated hydroxide ions. Sodium hydroxide is a multi-million-ton per annum commodity chemical. A hydroxide attached to a strongly electropositive center may itself ionize, liberating a hydrogen cation (H), making the parent compound an acid.
Oxide
The oxide ion has the formula O. It is the conjugate base of the hydroxide ion, OH and is encountered in ionic solids such as calcium oxide. O is unstable in aqueous solution − its affinity for H is so great (p"K" ~ −38) that it abstracts a proton from a solvent HO molecule: