That depends on the route.

The OCP pass is valid to Basel SBB and Geneva (all trains).

According to SNCF, it's also valid to Barcelona, Luxembourg, Milan and Freiburg(Breisgau) (with the TGV only).

You can use it on the TGV to Brussels (not on Eurostar) and TGV/ICE to Germany with the "Pass 2” fare. For Brussels, this is only available at the ticket office, but it has the same price as the “Pass 1” fare for global passes. TGV/ICE to Germany can be booked here:

https://int.bahn.de/en/buchung/start?KL=2&ET=PASSZUSCHLAG

(Choose the fare "Passzuschlag One Country Pass”)

I suspect the OCP pass is also valid to Ventimiglia and Portbou, in the latter case only with the SNCF TER.