##\mathrm{OH^-}## participates in the autoprotolysis of water.
$$\mathrm{2H_2O(l)\rightleftharpoons H_3O^+(aq)+OH^-(aq)}$$
Now, the definition of a Bronsted-Lowry base is a compound that is a proton acceptor.
##\mathrm{OH^-}## accepts a proton and is thus a Bronsted-Lowry base.
So, why do we...