The kind of CD-R matters as far as compatibility. I've heard also that burning one at slower speeds might help. CD-RW's are a crapshoot, so stay away from them. Far as the best I've heard TDK brands get high marks.
1. Burn at a lower speed, it will take longer but tends to work with finicky drives and discs. More like 4X. Theoretically, a disc is a disc but avoid CD-RWs. The reason is that they must be FORMATTED.
2. Research the brand you are buying.
3. If you are asked, choose to 'close the session' (i.e you may be offered an option to leave it open to add more (UDP?) DON'T))
4. Choose the option to check the disc before closing and ejecting if you have that, it will tell you if the write failed in any way and allow you to adjust burn settings better.
Your Christmas CDs I made for this group over the years often used Staples brand CD-Rs and I had to adjust as I stated above to create them. That was a time consuming process to say the least and one I would repeat for friends and family. In the end I just pointed many of them to our Dropbox downloads and told them how to make the insert card from the artwork. Printing the cards was stinking easy.
Nowadays I tend to burn a master to record a cassette if I want to send more than one to my friends as well as my own.
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.
Foobar 2000 is 100% free music player-format converter-CD Ripper software and it will play any format known to man. If you have a format problem it may help with that a bit as you can convert formats with it. IE FLAC to MP3
Audacity is free, been around for years ... and does a good job of converting formats. Once you get all the files in the wave format, you can use Windows Media Player to burn to disc... but it appears you already knew that.
If you're willing to throw down some hard earned coin ... you can't go wrong with Nero for ripping, converting & burning discs.
It goes on sale @ Amazon frequently and you can save some of that coinage by going with a version or two older than the latest... they still work fine on Win 10.