I'd first try a different set of interconnects.
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These kind of things are challenging to figure out. I'd suggest putting energy into trying different things to isolate where exactly the problem is. Mofimadness suggest is excellent. If you still get a buzz then move onto something else like:
- try plugging the rca cables into a different input in your preamp.
- try using a different output from the Oppo, like the digital out
- try plugging the Oppo into a whole different system