I use the HDMI cable that came with my DV-981. I really doubt I'm missing anything, as it works flawlessly. The only reason I can see for buying another cable is if the supplied one is too short, or defective in some way (In which case I bet Oppo would replace it for free).
If you are using analog multichannel interconnects, you could possibly hear a slight change in sound when using superior cables (not those fancy $100/ft. cables either), but when using HDMI, no way.