The design of operating system for Wireless Sensor Network (WSN) deviates from the traditional operating system design due to their specific characteristics like constrained resources, high dynamics and inaccessible deployment environments. We provide a classification framework that surveys the state of the art in WSN Operating Systems (OS). The purpose of this survey is two-fold, one is to classify the existing operating systems according to important OS features, and the other is to suggest appropriate OSs for different categories of WSN applications, mapping the application requirements and OS features. This classification helps in understanding the contrasting differences among existing operating systems, and lays a foundation to design an ideal WSN OS. We also classified existing WSN applications, to help the application developer in choosing the appropriate OS, based on the application requirements. Summary and analysis, and discussion of future research directions in this area have been presented.