You should know that when Oxygen Builder is active, the WordPress theme is completely ignored.
So I guess it doesn't make much sense to keep a default WordPress theme or any other theme installed.
The original author created a blank theme to be used with Oxygen Builder.
The theme is called GHOST and follows the WordPress guidelines for theme development.
Here are some great reasons to install Ghost:
Being an empty theme there are the files that are strictly necessary to make sure that it is recognized as a theme by the WordPress core, and therefore that you can install it.
There is also a surprise...
...Included in the theme is the Flickity library, very useful for creating sliders and carousels in Oxygen Builder.
It installs like any other WordPress theme:
At this point you can delete all other installed themes.
You can download the Ghost theme from here: