We consider a single-input single-output (SISO) nonlinear system which has a well defined normal form with asymptotically stable zero dynamics. We allow the system's equation to depend on bounded uncertain parameters which do not change the relative degree. Our goal is to design an output feedback controller which regulates the output to a constant reference in the presence of constant unkown input disturbances. The disturbance vector fields satisfy geometric conditions which ensure that the system is transformable into the so called disturbance-strict-feedback form. The integral of the regulation error is augmented to the system equation and a robust output feedback controller is designed to bring the state of the closed-loop system to a positively invariant set. Once inside this set, the trajectories approach a unique equilibrium point at which the regulation error is zero. We give regional as well as semi-global results.