This paper addresses performance enhancement using disturbance observers (DOBs) for a class of hysteresis nonlinearities characterized by the Prandtl-Ishlinskii model. The proposed approach makes use of internal model-based estimation of exogenous disturbances. The synthesis of the DOB is formulated as an $H_{infty}$ weighted-sensitivity optimization for static output feedback (SOF) gain of a Luenberger observer. A linearization approach is then implemented to solve the rank-constrained (non-convex) constrained semi-definite program (SDP) for the (sub) optimal static gain. Simulation results indicate that the closed-loop tracking performance is indeed enhanced using the DOB for reference inputs at different excitation frequencies.