What is the difference between oxidation …

Valency is the ability to make bonds,valence  electrons are the electrons that participate in forming chemical bonds. For exampleCarbon has a total of 6 electrons . However, 2 of those electrons are in the core of the atom ( in the 1s orbital). The remaining 4 electrons are in the outer 2s and 2p orbitals. Since these 4 electrons are in the outer shell, they can participate in bonding so carbon has a valency  of 4.

Oxidation number is a number used to designate how oxidized an atom is in a compound or molecule. It is the hypothetical charge an atom would have if all of its bonds were completely ionic (rather than covalent). Really oxidation state is that allows us to track what is being oxidized or reduced in a chemical reaction by comparing the oxidation states of the reactants to those of the products.