Which Oppo do you have? I have the 80h. It is possible that you have accidentally programmed the player to always output multichannel in which case your receiver thinks it is already 5.1 even if only two channels contain actual information.
Try to put the player in LPCM and choose "Stereo" as "Downmix". If that works, you can start trying to find a way to make it all work together.
If it is a BDP-83, make sure you have all the latest firmware upgrades. The beta reports from the field were mentionning quite a few issues with regular CDs. Since it is of lower priorities for a BD player, it is possible that glitches were found and fixed after the player was released.
