Custom PHP and HTML can be added to your page using the Code Block element. PHP behaves just like it does if you were coding it in a WordPress theme file. You can run the WordPress loop, make database queries, and use any other WordPress API functions.
To add a Code Block, click +Add > Basics > Code Block. Click PHP & HTML to access the editor.
Your code will be executed inside Oxygen, allowing you to see the results of your code live, without saving, uploading, and refreshing.