As per the Classical concept,
Oxidation is Defined as, the addition of Oxygen or any other electronegative element to a substance.
2Mg(s) + O2(g) → 2MgO (addition of oxygen)
Mg(s) + Cl2(g) → MgCl2(addition of electronegative element, chlorine)
As the removal of Hydrogen or any other electropositive element from a substance.
2H2S(g) + O2(g) → 2S + 2H2O(l)(removal of hydrogen)
2K4(aq) + H2O2(aq) → 2K3(aq) + 2KOH(aq)(removal of electropositive element, potassium)
As per the Electronic Concept,
Oxidation involves the loss of one or more electrons from an atom or an ion. It leads to an increase in positive charge and decrease in negative charge.
Fe2+ → Fe3+ + e-(increase in positive charge from +2 to +3 for Fe)
2Cl- → Cl2 + 2e-(decrease in negative charge from -1 to 0 for Cl)