How is valence different from oxidation state?

Valence is the ability of the atoms of an element to attach a certain number of atoms of another element. The valence unit is taken as the valence of the hydrogen atom.
The oxidation state is the charge that an atom acquires as a result of the donation (+) or attachment (-) of electrons. The oxidation state and valence can be numerically equal.
For example: CaO – calcium valence 2, and the oxidation state +2, because he donated 2 electrons;
the oxygen valence is 2, and the oxidation state is -2, because he took 2 electrons.