Summary: This chapter gives a historical background on and shows the functionality and architecture of OASIS 1 . It also describes interactions of transmission providers and transmission customers with OASIS. In addition, the chapter shows how requests to sell and purchase transmission services are conducted through OASIS. Major types of information posted on OASIS would include available transfer capability (ATC) and total transfer capability (TTC), ancillary service offerings and prices, transmission service products and prices, transmission service requests and responses, facility status information, transmission service schedules, and transmission-related communications. ATC values are key to competitive electricity markets as they are indices that determine whether particular proposed transactions of electric power can or cannot be approved or occur. In this chapter, we define and explain the concepts of TTC, ATC and the related terms in detail. The chapter includes a few examples of OASIS network implementation in various regions of the United States. The presented examples show how the OASIS information could differ from place to place while some basic information is shared by various OASIS implementations.