1. Valency is the maximum no. of electrons which an atom can lose or gain while oxidation no. is the actual no. of electrons which the element lose or gain to form a molecule.
2. Valency is the property of an isolated atom while oxidation state is calculated for an atom is present in a molecule i.e, in a bonded state.
3. Valency of an element remains the same while its oxidation state may be different in different molecules.