PS
The above is assuming you have double checked the settings on the Marantz as well and told it video is coming in on one interface,an d audio on another. And that you have set the appropriate resolution and image size on the Marantz and Oppo as required.
The Marantz default could be set up differently if multi ch analog audio inputs are used in combination with video. It could be too, you’ll need to ‘sync’ the two inputs up later via the Oppo’s setting which provide for just such a situation. My old ONkyo Rec does this. I suspect your Marantz does too.