Possibly because Playbar only supports stereo PCM and DD5.1 via its optical connection.
It's worth checking if you have the Oppo set to Bitstream. If so, and you are playing BDs with Dolby TrueHD and DTS-MA audio, then the TV and the Playbar won't know what to do with that signal. Set the Oppo to down convert the audio.
Unless you need a 4K UHD DB player, then simply buying one to replace the Oppo isn't guaranteed to fix the problem since it's a settings issue rather than a hardware incompatibility. IOW, you'll have the same problems if the UHD player isn't set up right.
