Dear student
Addition of oxygen or removal of hydrogen from the reactants is oxidation.
In the reaction below, on addition of oxygen, hydrogen is being removed from hydrochloric acid and chlorine is formed.
Hydrogen chloride can be oxidised to chlorine by the Deacon process:
4HCl(g) + O2(g) --> 2Cl2(g) + 2H2O(g)
Regards