Oxygen eliminates the need for child themes.
Everything about the design of your website is stored in the WordPress database. You can safely update Oxygen without affecting your design and customizations.
Need to put some custom PHP functions somewhere, and using an Oxygen Code Block isn't appropriate?
Don't use functions.php. Instead, put your functionality in a plugin.
This way, you don't need to use a child theme, and updating Oxygen won't overwrite your custom PHP.