You only get the 'core' 5.1 (an Atmos decoder is required for the height information), but I think it's great that folks with legacy 5.1 setups can still hear all these newly-mixed surround titles exclusive to Apple's streaming service. Tom Petty's
Wildflowers still sounds fantastic without the height channels.
I don't think the Oppo units (or any disc players for that matter) have HDMI inputs, but connecting the Apple TV 4K directly to your AVR via HDMI would probably get you 5.1 DD+ or DD. If your AVR or Amp is old enough to pre-date HDMI, you can use one those HDMI-to-analog boxes that I linked in my original post.
Unrelated, but I also found that the VLC Media Player app for Apple TV 4K will play back 4.0 & 5.1 FLACs via a NAS drive