It’s not in Discover, and not in Settings-Themes-Global Get New.
I know its currently an optional package under in most distro’s repos. I suppose it might be technically possible to package it as a systemd-sysext, but that’s a royal pain in the bum. Maybe if you ask Nate very nicely he will consider adding it.
@Trog-Troginson I already raised the possibility with Nate and we’re not going this way. Decision has been made to not include the theme because it would take a large amount of space on the images.
It is first-party (so not in Get New / KNewStuff), if you want to get it you’ll unfortunately need to build it yourself with kde-builder.
To build it (Not recommended for inexperienced users)
I don’t recommand because it will overlay Oxygen and all its required dependencies on top of the image so you’d need to take care of maintaining the system extension to not be out of sync with the base.
Follow this guide and then run kde-builder oxygen oxygen-icons oxygen-sounds
In the long-term, the goal is to get Union in a state it would be possible to port or recreate Oxygen as an Union theme.
Is it really that big? Interesting. Yeah, Union is the future, so if you can make it work with that, then that would be great. I am currently testing the new Oxygen theme on Fedora Kinoite. Its great.
Trying to use it with union results in a lot of weird jank. Clearly needs some work to get all that up to snuff.
OP here. Huh - here I thought the OP got the first crack as marking a post solved! 
Well, I do agree, it is solved.
I just wanted to compare Oxygen Dark to the Breeze Dark I’m currently using. But, because KDE Linux is an alpha, I’m also running Garuda Linux, and will check Oxygen Dark out there when Garuda gets to Plasma 6.7.
Oh, if you look on your notifications, Nate himself marked it as solved (he’s a mod here so he can haha), but usually yes, OP is the one marking as solved.
That is a real shame. I use Oxygen on my FD44 KDE Plasma 6.7.1 and it gives some style to the system. At the moment, KDE-Linux Desktop looks really flat, even after some theme adjustments.