OPC is a series of standard specifications.
The first standard (originally called the OPC Specification and now called the Data Access Specification) resulted from the collaboration of a number of international automation suppliers working in cooperation with Microsoft. Originally based on Microsoft's OLE COM (component object model) and DCOM (distributed component object model) technologies.
The specification defined a standard set of objects, interfaces and methods for use in process control and manufacturing automation applications to facilitate interoperability. The COM/DCOM technologies provided the framework for software products to be developed.
There are now hundreds of OPC Data Access servers and clients.