This doesn’t seem to be in the Guide etc…
Unplug everything and turn off all. Turn on TX-6. Set TX-6 USB to 12-in. Plug everything in. Turn on OP-1. Press play. Audio appears on TX-6 Ch1, with the advantages that gives in terms of EQ, FX etc.
This means you can use the line out from the OP-1 to feed an external FX setup. I’ve got it going into a headphone splitter with volume per channel on it, feeding four pedals, which then go into the analogue ins on the TX-6. Then you still get further processing options via the Aux send, and channel strip controls too, on everything.