An adaptive output regulation design method is proposed for a class of output feedback systems with nonlinear exosystem and unknown parameters. A new nonlinear internal model approach is developed in the present study that successfully converts the global robust output regulation problem into a robust adaptive stabilization problem for the augmented system. Moreover, an output feedback controller is achieved based on a type of state filter which is designed for the transformed augmented system. The adaptive control technique is successfully introduced to the stabilization design to ensure the global stability of the closed-loop system. The result can successfully apply to a tracking control problem associated with the well known Van der Pol oscillator.