Oxidation number vs charge?
If an atom loses electrons, it develops a positive charge and becomes a cation. Likewise, if an atoms gains electrons, it develops a negative charge and becomes an anion.

"Charges only EXIST on IONS."

Covalent bonding rose the oxidation number.
We are aware that in a Covalent Bond sharing of electron takes place.
The more electronegative atom will take more bonding electrons and it will get a negative oxidation number (It depicts Anion).
Similarly less electronegative atom mimics a Cation.

In other words you can say Oxidation Numbers are fictional ionic charges