Because Oxygen is not a theme there is no functions.php file.
If you need to put some custom PHP functions somewhere, and using an Oxygen Code Block isn't appropriate, 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.
Here is an example plugin you can use as a starting point for your own: https://github.com/srikat/my-custom-functionality
