This is a free plugin for the community!
Oxy Dev Wall lets you build Oxygen templates behind the scenes, and only shows Oxygen to signed in admin users. Unregistered, non-admin users will see the site as generated by the theme or other builders. Only you and other admins will be able to build your Oxygen templates, apply them, and view them on the front end. Changing any content will also be visible on the front end for all users, so this really helps with templating and design ONLY.
Download the ZIP from GitHub: https://github.com/emzimmer/oxydevwall
I based this plugin on similar operations to the Optimizer extension. It generates a file in the must use plugins directory (mu-plugins) inside wp-content. When the plugin is deactivated, that file will be deleted. It prevents Oxygen from loading at all for non-signed in visitors. To view Oxygen built templates and designs, it checks whether you're an admin and signed in first.
This plugin is fairly simple. There are no settings or admin pages to worry about. It will not adhere to your Oxygen client control settings, so if you need some complex accessibility you will want to update the privileges as mentioned in the Readme file.
The plugin is provided on an as-is basis for free to the Oxygen community. It is up to you to test it and make sure it works as you expect for the purpose described here. In other words, test it first to make sure it works with your configurations and settings.