Theme
There are 3 reasons I can think of as to why you should use a barebones (blank) theme for Oxgen Builder:
i) To stop annoying theme update notifications
ii) To look more professional as you can change the details as required to your client’s details
iii) Link the developer’s details for anyone inspecting the site
Plugin
There are 3 reasons I can think of as to why you should use a barebones functions plugin for Oxygen Builder:
i) To have a single repository for all (types) of custom code
ii) To look more professional as you can change the details as required to your client’s details
iii) To add more functionality to your website
Note: You are not limited to using your own custom theme and plugin on Oxygen! You can use it with the likes of Divi, Elementor etc.