Uh, one silly question… is the Marantz also 2K & 4K video capable via HDMI as is the Oppo?
If not, then right there might be the reason.
To check, try a DVD. Then a std BR. Then a 4K BR. This should provide more clarity as to who or what is the issue.
Which HDMI output to the Marantz is being used? The main or secondary?
If all else is well, IMO, Sounds like a handshake issue.
If so, power both the Oppo and marantz down. Unplug the HDMI cable feeding the Mary. Power the mary up. Power it off again. Plug in the HDMI cable from the oppo. Power up the oppo. Power up the Mary. Select the appropriate HDMI input on the Mary being fed by the oppo. Spin up a disc. it should work now.
Unless Oppo changed things, previous Oppo’s outputs have all been hot. So signals into the mary should be available.
One last note could be that as you are sending audio via one set of inputs and video thru another, if I read the note right, you might need to change a setting on the Mary, telling it what’s what and or where to look for it.. .