The OPC Interoperability session is an event the OPC Foundation hosts three times every year. During the event, OPC vendors gather to ensure their OPC Clients and OPC Servers can communicate with each other. Thus, Vendor X tries to connect their OPC Client application to vendor Y’s OPC Server and also the OPC Server by Vendor Z. The success and/or failure results are posted on the OPC Foundation’s web site.
You should ask all OPC vendors who state their applications are OPC compliant about the last time they attended an OPC Interoperability session and the results of the test.