The issue is definitely that you are trying to do something a bit out of the ordinary. The receiver itself may offer a few different options for the solution, but I think the best is simply to take the audio and video from the cable box and split them up.
Oppo HDMI Out --> Emotiva XMC HDMI DVD Input (or similar)
Cable Box HDMI Out --> TV HDMI Input 2
Emotiva XMC HDMI Output --> TV HDMI Input 1
Cable box optical audio output --> XMC Optical audio input
Now, you can put the TV on input 1 if you want to see the Blu-ray Disc, or you can put the TV on input 2 if you want to see the cable box.
You can put the XMC on the DVD input to hear the Oppo, or you can put the XMC on the 'digital audio' input if you want to hear the cable box.
This type of mixing and matching is possible, but it isn't really straightforward as you do have to deal with separating your sources and creating new pathways for audio.
If your cable box does NOT have a optical audio output, then you can use the optical audio output from your TV, or pick up a HDMI audio extractor on Amazon to create that connection point.
Like this:
https://www.amazon.com/J-Tech-Digital-Extractor-Passthrough-JTD18G-H5CH/dp/B074HHSJVN/ref=sr_1_2_sspa