Here are a few ways in which you can change the design/reset your site when using Oxygen.
Choose an installation type
If you wish to get back to the page that comes up as soon as Oxygen is activated, go to
example.com/wp-admin/index.php?page=oxygen-vsb-setup
From here you can either install the default premade website (Atomic) or select another by browsing the library or do a totally blank install.
Your existing content should remain.
Choose a Design Set
In your WordPress dashboard, go to Oxygen > Home. Then click on “Install a Different Website” link to get to a page that shows all the various pre-built sites’ design set.
Here you can click on Install for your desired design.
Your existing content should remain.
Reset database
Use a plugin like WP Reset to reset the WP database back to the default installation values without deleting or modifying any files.
All the content will be erased.