I'm not an 'expert' on file formats, but I have investigated them in the past, and I know what sounds good to me. If any of the technical info I give you isn't quite correct, I hope someone will chime in, although I doubt the information would be radically wrong.
As I remember, .ogg isn't really an audio format by itself; it's a *container or framework* for an audio codec. There are several that are used with .ogg, the most notable was Vorbis, which I think is a separate project. Its value is that it can stream several media streams at once, like video+audio+text. It is used more in browsers than in dedicated music players, I think.
Ogg+Opus isn't a particularly high-quality combination; it's a lossy format primarily for spoken word and low-quality audio, because it has a fairly low [although variable] bitrate. I'm not sure what its highest bitrate is, but iirc its purpose isn't to save your audio collection forever.
I don't think that would be the combination you'd want for your music, but if space is an issue, I could understand it.
I think it would be better to figure out how your particular app would make Ogg+FLAC, [I'm pretty sure that can be done] because the *primary purpose* of FLAC is to make archived copies of music CDs.
But for you, Dan, your ORIGINAL choice of format is the most important; what did you use? If it was ogg+vorbis or ogg+opus, you encoded to lossy formats. If you are lucky, you ripped your CDs to ogg+PCM, which is a non-lossy format that is similar to the original .wav files on the CD, like Chris told you in his post. Then, you have all of the bits, and you could convert to FLAC, which is really, really where you want to go. [But I don't think you did this, from what you've said] If you used .ogg+opus, what bitrate did you convert to? You can probably find out in the 'properties' metadata in the files.
So the big question is: when you ORIGINALLY ripped your CDs, what format did you use [remember, ogg isn't an audio format, it's a container within which an audio format operates]
[You might have a new hobby. 300 CDs aren't really THAT much now, are they?

Rip directly to FLAC, you won't be disappointed]