Why does $ O{H^ - } $ be an ion?

Hint: Atoms combine with each other and form molecules when a molecule or an atom bearing a charge of one or more than one can be known as an ion. Water on dissociation produces two ions namely hydrogen ion and hydroxide ion where the hydrogen ion is simply known as a proton.

Complete answer:
Atoms consist of three subatomic particles like electrons, protons, and neutrons. Electrons are the negatively charged subatomic particles, neutrons are the neutral or zero charged subatomic particles, and protons are the positively charged subatomic particles.
Hydrogen is an element with atomic number $ 1 $ and has only one electron and one proton. When hydrogen loses one electron it becomes an ion with a positive charge known as hydrogen ion, as this hydrogen ion has only one proton with zero electrons and zero neutrons. It can be simply known as a proton.
Atoms or molecules bearing one or more positive charges are known as ions. Water one dissociation produce the ions as follows:
 $ {H_2}O \rightleftharpoons {H^ + } + O{H^ - } $
Where $ {H^ + } $ is hydrogen ion
 $ O{H^ - } $ is hydroxide ion
The oxygen and hydrogen atoms combine to form a hydroxide ion as the oxygen is more electronegative the negative charge is associated with oxygen.
Thus, the hydroxide group $ \left( {O{H^ - }} \right) $ is known as ion.

Note:
The ion that bears a positive charge is known as cation and the ion that bears a negative charge is known as an anion. In the above dissociation reaction, a hydrogen ion is known as cation and the hydroxide ion is known as an anion.