Oxidation Reaction according to the Classical Concept
Oxidation is Defined as, the addition of Oxygen or any other electronegative element to a substance.
Example,
2Mg(s) + O2(g) → 2MgO (addition of oxygen)
Mg(s) + Cl2(g) → MgCl2 (addition of electronegative element, chlorine)
Or
As the removal of Hydrogen or any other electropositive element from a substance.
Example,
2H2S(g) + O2(g) → 2S + 2H2O(l) (removal of hydrogen)
2K4[Fe(CN)6](aq) + H2O2(aq) → 2K3[Fe(CN)6](aq) + 2KOH(aq) (removal of electropositive element, potassium)
Oxidation Reaction according to 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.
Example,
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)

Related Chapter - Common Types of Organic Reactions