Actually, since the Oppo is a DVD player (I am assuming is not a BR player), then you don't have HD audio from that player anyway - HD audio codecs are only BR discs. IMO I would just use the BR player for everything - the only thing you'd be giving up is the Oppo's well known good up-conversion for standard def video material - but this will not affect your audio options. Unless SACD or DVD Audio are a concern for you? Those might be other reasons why you'd want to keep the Oppo in the mix.
So, the only way you're going to get HD Audio with your current setup is to use LPCM output from the BR player over HDMI to your receiver.
Hope that helps, and if I've made a mistake or bad assumption please correct me!
