By default, access to Oxygen is only available to administrators. Access to Oxygen can be granted to other user roles at Oxygen -> Settings -> Role Manager in the WordPress admin.
Oxygen's role manager works with the WordPress default user roles, any additional user roles added by custom code, and any additional user roles added by 3rd party plugins such as User Role Editor.
Important Security Information
Access to Oxygen should only be granted to trusted users. Because Oxygen provides the ability to execute PHP code, any user granted access to Oxygen could run PHP code to make themselves an administrator.