There are quite a few relationships between [H+] and [OH−] ions. And because there is a large range of number between 10 to 10-15 M, the pH is used. pH = -log[H+] and pOH = -log[OH−]. In aqueous solutions, [H+ ][OH- ] = 10-14. From here we can derive the values of each concentration.
-log[H+ ] + -log[OH- ] = -log[10-14]
pH + pOH = 14
So pH = 14 – pOH and pOH = 14 – pH