I think it's ironic that it's argued above that USB done right is awesome...in context of converting it back to S/PDIF :)
To the OP, yeah, two paths - one would be a USB-SPDIF converter from Mac to Oppo (loads of them around now). Another would be to double-down on improving the USB from the Mac with stuff like iFi iUSB, iFi iPurifier, a better USB cable.
It still comes down to what the Oppo seems to do better with. Try to do some comparisons across the different inputs to see if you can detect the baseline preferences it may have between optical, coax and USB.
To the OP, yeah, two paths - one would be a USB-SPDIF converter from Mac to Oppo (loads of them around now). Another would be to double-down on improving the USB from the Mac with stuff like iFi iUSB, iFi iPurifier, a better USB cable.
It still comes down to what the Oppo seems to do better with. Try to do some comparisons across the different inputs to see if you can detect the baseline preferences it may have between optical, coax and USB.