This paper describes the use of continuous observation of oscillatory stability to validate the accuracy of a dynamic model of a real power system. It is shown that there is a correspondence between the observed and modeled dynamic performance but that there is also some significant differences, including some observed cases of degraded damping not being reflected in the model. The differences between observed and modeled dynamics provide useful information to prioritize more detailed plant-based testing for model validation. Furthermore, real-time measurement of dynamics can be used in system operations to improve security where there is uncertainty in the model-based evaluation of the stability of the system. The approach presented in this paper makes use of much more detailed information on system dynamics that can be extracted from WAMS, leading to improved system modeling. Improving the model validation process can result in significant increases in transfer capability or system security.