Are you playing CD, DVD, BR, streaming, or trying to feed the CATV signal thru the Oppo into your DAC?
Apparently you want to feed from the DAC too as you have a USB connection coming from the DAC to the 103.
Does neither scenario work? Feeding the 103, or using it solely as a disc spinner?
Every Oppo I’ve had just connected to whatever DAC via TOS or COAX and played without issues.
Incoming info on USB might require some Settings manipulations.
A simple thing to try is while playing into the DAC, press the AUDIO button on the Oppo remote control. This will cycle thru some of the various audio formats available during DVD or BR playback. It will default back to what it was before when you stop [playback and remove the disc. Or shut it down. I don’t thing it applies when streaming. Dunno.
The MENU button on the Oppo remote should put you into Settings. Previous models also allowed some of the info to be displayed on the Oppo front display screen.
If a LCD is not readily available unplug it and find one. Press the menu and look for digital output settings as has been said, PCM is what you need to find as output format.
You can Google for this issue. One can always call Oppo too.
Make sure to post back with the solution, your particular Oppo model.
Good luck.