An ocean observing system could be defined as a set of independent elements that interact to form a whole for the purpose of observing ocean data. But, what is a system? Is a sensor a system? Is a buoy a system? Is a collection of stations a system? This paper defines the components of an ocean observing system and how they relate to each other for the purpose of streaming real time sensed data. This paper answers these questions based on results from the Open Geospatial Consortium (OGC) Ocean Science Interoperability Experiment. The paper also provides a conceptual model of an ocean observing system and web services examples based on OGC Sensor Observation Services (SOS) and the IEEE 1451 set of standards. The OOSTethys group, leader of the OGC Ocean Science Interoperability Experiment, has been building a community of interest, developing reference implementations, and maintaining a test-bed environment to advance the OGC Sensor Observation Service (SOS) and IEEE 1451 standards. The primary goal of the OOSTethys project is to accelerate the pace at which ocean observations and associated technologies become more broadly and publicly available by expediting the community-wide adoption, installation, and updating of standards-compliant web services.