How to apply Le Chatelier's principle?
Remember that $\ce{HCl}$ (aq) and $\ce{H2O}$(l) are liquids while the oxygen and chlorine are gases . As the number of moles of gases on the left  hand side (one mole of oxygen) is smaller than that of the right hand side (two moles of chlorine), decreasing the pressure makes the equilibrium position move to the right hand side, therefore the concentration of chlorine increases.