As long as they support the Fidelio FIAS (Fidelio Interface Application Specification) protocol, it will work. You should confirm with them that this version of Opera supports this.
Yeah, we have it working with Opera Cloud at one client, although we still have a local interface machine that 3CX talks to and that talks to Opera Cloud. Same machine handles other local interfaces as well.
Yeah, all you should need to do in 3CX is update the IP to point to the interface machine (the installer will give you a port, like 5010) and away you go. The installer will use the ifc_controller on the machine to receive from 3CX and transmit to Oracle cloud.