New users of Oxygen Builder who are looking to place some string of code inside the functions.php can get confused easily. Is there no functions.php? The short answer is NO. Oxygen disables any themes so there is no functions.php.
Does that mean you can't add any code? Here the short anwer is YES you most definitely can.
Inside the Oxygen Builder you have the option to add a Code Block. But sometimes that is not the solution you are looking for. In that case there is always the Code Snippets
plugin, which is a great plugin that offers many uses.
You can add your snippet to one of these.
For a more indepth explanation of the use of Code Blocks, I point towards this video.