We can assume that the concentration of H+ is 0.26M, because we are given [H3O+] and it is a strong acid.
So we will calculate the pH to obtain the pOH. Then apply pOH =-log[OH-]
pH = -log[H3O+]
pH = -log[0.26]
pH = 0.585
Then, we know that pH + pOH = 14
re-arrange:
pOH = 14 - pH
pOH = 14-0.585
pOH = 13.415
Now we can calculate the [OH-]
pOH = -log [OH-]
[OH-] = 10^-pOH
[OH] = 10^-13.415
[OH-] = 3.846x10^-14 M