In the audio set-up of your Oppo BD player, you might have the preference set to a lower setting. My Panasonic has a setting where it downconverts HD audio to Dolby Prologic (something to do with multible audio tracks). Make sure your player isn't doing something like that. Ideally it should be outputting 7.1 channel LPCM, Dolby True HD, and DTS HD MA to the Receiver through the HDMI cable, or Bitstreaming the HD audio to the Receiver for it to decode.

Also, if your audio is being run to the receiver through anything less than an HDMI cable, you are NOT getting HD sound. Coaxial cable, RCA cables and optical cables, though good, are not capable of transferring the full uncompressed, lossless audio.