OPC applications can communicate with each other even when they are on different Windows Domains. The trick to getting communication working is Windows Authentication on both the OPC Client and OPC Server PCs.
First, the OPC Server PC must recognize the User Account of the OPC Client PC. Therefore the User Account of the OPC Client must exist in the Active Directory (located on the Domain Controller) of the OPC Server PC. Alternatively, the OPC Server PC must have a local User Account setup for the OPC Client application.
Second, the OPC Client PC must recognize the User Account of the OPC Server PC. Therefore the User Account of the OPC Server must exist in the Active Directory (located on the Domain Controller) of the OPC Client PC. Alternatively, the OPC Client PC must have a local User Account setup for the OPC Server application.
All this can be made easier if the IS (Information Systems) or IT (Information Technology) staff setup a Trust between the two Windows Domains. This will ensure that all User Accounts will be automatically synchronized between the two Domains.