Are you using DLNA (network connection) to the Oppo?
If so, your choice of DLNA renderer profile (within MC) will affect this. It's kinda complicated, so I'll leave the details to someone else. If this does turn out to be a DLNA thing, you might want to repost this in the Media Network forum because there are people there that are very good at DLNA. Very few of them hang out on the Mac forum.
Good luck!
Brian.