An ocean observing system could be defined as a setof independent elements that interact to form a whole for thepurpose of observing ocean data. But, what is a system? Is asensor a system? Is a buoy a system? Is a collection of stations asystem? This paper defines the components of an ocean observingsystem and how they relate to each other for the purpose ofstreaming real time sensed data. This paper answers thesequestions based on results from the Open Geospatial Consortium(OGC) Ocean Science Interoperability Experiment. The paperalso provides a conceptual model of an ocean observing systemand web services examples based on OGC Sensor ObservationServices (SOS) and the IEEE 1451 set of standards. TheOOSTethys group, leader of the OGC Ocean ScienceInteroperability Experiment, has been building a community ofinterest, developing reference implementations, and maintaininga test-bed environment to advance the OGC Sensor ObservationService (SOS) and IEEE 1451 standards. The primary goal of theOOSTethys project is to accelerate the pace at which oceanobservations and associated technologies become more broadlyand publicly available by expediting the community-wideadoption, installation, and updating of standards-compliant webservices.