OPC is a series of standards specifications. The first standard (originally called simply the OPC Specification and now called the Data Access Specification ) resulted from the collaboration of a number of leading worldwide 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.

Everyone's favourite analogy for needing the original Data Access Specification is printer drivers in DOS and then in Windows. Under DOS the developer of each application had to also write a printer driver for every printer. So AutoCAD wrote the AutoCAD application and the printer drivers. And WordPerfect wrote the WordPerfect application and the printer drivers. They had to write a separate printer driver for every printer they wanted to support: one for an Epson FX-80 and one for the H-P LaserJet, and on and on. In the industrial automation world, Intellution wrote their Human Machine Interface (HMI) software and a proprietary driver to each industrial device (including every PLC brand). Rockwell wrote their HMI and a proprietary driver to each industrial device (including every PLC brand, not just their own).

Windows solved the printer driver problem by incorporating printer support into the operating system. Now one printer driver served all the applications! And these were printer drivers that the printer manufacturer wrote (not the application developer). Windows provided the infrastructure to allow the industrial device driver's solution as well. Adding the OPC specification to Microsoft's OLE technology in Windows allowed standardization. Now the industrial devices' manufacturers could write the OPC DA Servers and the software (like HMIs) could become OPC Clients.