Even if this is an old topic, others may look for the same solution, as I did.
OPC used to be a Windows standard, but it has changed and a lot of tools,
including OPC server, exist for Linux based system. You could find much of
those here :
Unfortunatly, most of them are propriatary (all of OPC and SDK there, are).
But I found this :
I didn't test it yet, but Rasperry Pi compatibility is specify, and it is
supported by a lot of big companies.