Hello, long time lurker here, I’ll post at some point about my projects involving the wonderful Zynthian, in the meantime I just wanted to stress that the OP-Z can be connected as a USB sound interface, i.e. you can indeed output the Zynthian audio to the OP-Z with USB only, no need for the TRRS audio cable. At least one other person did the same, see A couple of headless problems (with the op-z)
You have to configure the Zynthian to use a USB soundcard. I don’t have the Zynthian up and running right now to give you the exact parameters to set in the web interface, but I did not have troubles to switch back and forth between the OP-Z and my regular USB audio interface (I don’t have anything beside that to get the audio), the OP-Z behaves as a regular device here (and the MIDI is of course still working).