Time-varying unknown wind disturbances significantly influence the dynamics of the wind turbines. In this paper, a disturbance observer based (DOB) control scheme is illustrated to reject disturbances to the wind turbines at above-rated wind speeds. Specifically, we aim at maintaining a constant output power and achieving better generator speed regulation when the wind turbine is operated at time-varying and turbulent wind fields. The inner DOB scheme is augmented with a collective gain-scheduling proportional integral (PI) feedback controller to asymptotically reject the persistent unknown time-varying disturbances. The proposed algorithm is applied to both linearized and nonlinear National Renewable Energy Laboratory (NREL) offshore 5-MW baseline wind turbines. Also an extra compensator to deal with the mismatch between the linearized reduced-order model and the actual nonlinear wind turbine is discussed. The novel application of the augmented DOB pitch controller demonstrates improved power and speed regulation in the above-rated region both for linearized and nonlinear plants.