Give OPPO a call and they can help you sort this out. The ROKU 3 should have no problem with the 103's HDMI Input. Be sure to tell OPPO what firmware you have on your 103:
Setup > Device Setup > Firmware Information > Main firmware version number.
The HDMI cables you are using should be "high speed" cables. I.e., sold as "For 1080p", or "High Speed", or "Category 2", all of which mean the same thing. This includes both the cable coming from the ROKU and all the rest of the HDMI cables in your system (HDMI is an "end to end" protocol). HDMI plugs are only friction fit, and it just takes a small shift of plug in socket to screw things up. So check that both ends of every HDMI cable are fully inserted STRAIGHT into the socket, with nothing (e.g., cable weight) tugging on them.
While waiting to reach OPPO, also try the HDMI FRONT Input on the OPPO. If the ROKU works OK with that but still not with the HDMI BACK Input then your 103 may need service.