WordPress nerds, rejoice. Design and functionality are separate.
A well established principle of website and software development is that design and functionality should be separate. In the context of WordPress, that meant that your theme should control your site's design, and plugins should control your site's functionality.
For the last few years, themes have been including functionality that should have been in plugins, resulting in one of the biggest complaints in the WordPress world: bloated themes.
Oxygen solves this. With Oxygen, you can control the design of any plugin. So there's no need to include functionality that is correctly left to plugins inside Oxygen.
Just install the plugin of your choice, and then make it look great with Oxygen.
Finally, things are back to the way they should be.