Streaming - ? To my old fashioned stereo.

Oppo has a Tidal app.  I suggest you subscribe to Tidal and then use Oppo for it.

Considering OPPO already supports streaming functionality it should be pretty easy... (you might need to update firmware on 205 by the way) 

Oppo looks like its already Roon end point - so if you subscribe to Roon that would do it. Or Tidal as was already suggested... 

Benefits of Roon is integration of different streaming services as well as access to any local music file library you already have or might want to have in the future. Plus it offers better metadata, dsp functions and ability to stream to multiple end points (multi-room). Minus is that its limited to your home and you can not use it outside ( at least not without making it complicated)  If you just want to stream from the web - Tidal is excellent and an easy solution in your case. 

You can always add external streamer - not that I know why would you want to :)  but.... - cheapest option would be Raspberry PI based one - its a DIY type but its all very easy .
More commercial and polished solution would be to get something like Bluesound Node 2i...

You can also stream to OPPO straight from your phone/tablet with mConnect Player... You would have to check if connect support Stigray music... it does support all main high res streaming services. 

Thanks.
Erik. What is the simplest way for me to get Tidal to the 205?

Honestly not sure, since I stream everything from Roku or Roon these days, but it was the same place you'd find Netflix.  Perhaps under network or apps??

Look through the top menu.