I think I see the problem. does the Oppo play mp3s? I don't think FLACs are generally played by most players and require a decoder program on your computer.
You need to convert them to a format your player recognizes and then burn the disc. Generally speaking that is plain old CD audio and newer players may play mp3s natively but that seems to be more of a portable player thing. You can archive them any way you like but you need to burn in a format the player recognizes.
Download AUDACITY. You'll have to follow the instructions to download the LAME software for mp3s because of licensing issues that would prohibit Audacity from being free software but they explain this on the site and in the instructions (which I do recommend you download with the program).
How do you think I make all those Christmas albums for Phoenix? BINGO.
Go here:
https://www.audacityteam.org/download/
That is the one legitimate source for the
free software and there are versions for Mac and Linux as well.