I've had a bit of a struggle getting a working test going, but the support team at Software Toolbox are excellent!
I hope to leave a very detailed log of my successful implementation for others to benefit from. Google'ing "OPC XOJO" will not return anything useful, so forgive me if I embellish this thread with TMI. First a quick recap...
Objective: Create Windows-based custom software that can Read/Write process variables and set points within the DeltaV environment via OPC-DA (COM), "Classic" specifications, thru Microsoft COM (OLE automation).
OPC Client Toolkit (drivers): ‘OPC Data Client’ from OPC Labs (Czech Republic), sold and supported by Software Toolbox (Charlotte, N.C.)
OPC-DA clients use a ProgID (Program Identifier) to specify the OPC Server they want to talk to, and then ask the server for a specific tag (piece of data from the field device). They can either do a one-shot read, or ask the server to send it updates any time the data changes in the field device.
The version installed and tested: OPC Data Client Version 2017.1 (5.50) was released on August 2nd, 2017
In order to use ‘OPC Data Client’ in the XOJO environment, all development and deployment must be done on a Windows platform, where OLE is supported.
XOJO IDE used for testing: version 2017 release 2
OPC Data Client User's Guide and Reference: