Assuming your Oppo has the same digital outputs as the Oppo BDP-105, its digital coax output (and likely its digital optical output as well,) passes WAV & FLAC signals via s/pdif. In theory you should be able to play any recording that's captured in WAV or FLAC format though those two outputs - the stipulation being the signal-receiving limitations of your preamp, or your integrated amp, or whatever piece of stereo gear that's on the receiving end of the digital signal. Many older preamps and integrated amps are incapable of accepting incoming digital signals with high resolution rates.
Save for watching movies via my DVD players my experience w/ HDMI is pretty limited. But I do have quite a few commercially released (?) DVD-Audio discs & have never encountered any issues playing them via my BDP-105 > coax digital cable > Anthem integrated amplifier, or via any of the other optical disc players with DVD-A playing capability I've owned in the past. I only listen to stereo layer of my DVD-A discs though, and have never tried play the 5.1 layer on the ones that have them. Perhaps your DVD-Audio discs aren't currently playable via your Oppo's digital coax output is because the unit's digi output is set to 5.1? Could also be that the piece of stereo gear on the receiving end of the digital signal can't do 5.1. (Could also be both issues.) If haven't done so already, perhaps take a look at how those two pieces of currently configured for outputting & receiving the 2.0 (the stereo layer) of DVD-A discs via their respective digital coax output & inputs.
Glad you brought this up. I'm checking all my SACD's. Slowly. Most appear to be hybrid. What I do know is the DVD Audio discs I have do play thru HDMI but not coax or optical. Same with BluRay audio. I guess that makes sense.
Are the SACDs you mentioned hybrid discs (they have both a SACD layer & a CD layer) or do they have just the SACD layer? I ask as I wasn't aware the signal from SACD could be carried over HDMI.
I listen to all my SACD's, DVD Audio and Blu Ray audio on an OPPO BDP-93>HDMI out>Yamaha 5.1 receiver and burned DVD's with 24/48 or 24/96 files on an OPPO BDP-80>SPDIF out to integrated 2 channel amp at full resolution. Also play regular CD's on the BDP-80. No problems on either setup.