If the Oppo does not have analog out for surround channels, you need to get an HDM 5.1 analog converter.
http://www.amazon.com/ViewHD-Extractor-Decoder-Headphone-VHD-H2HADi/dp/B00L0XIHEC/ref=sr_1_3?s=electronics&ie=UTF8&qid=1436615795&sr=1-3&keywords=hdmi+5.1+analog+converter
I use one I bought from Amazon to plug the surround channels into an old surround receiver without HDMI, and it works perfectly. The HDMI is a pass through so I am able to plug directly into the projector.