Security Considerations

Shortcode Signing
At Oxygen -> Settings -> Security you can choose to prevent Oxygen from executing its shortcodes if they lack a valid signature by selecting "Check Oxygen's shortcodes for a valid signature before executing". As of Oxygen v3.2, an invalid signature warning will only be shown in the WordPress admin area and not on the front-end of the site.
To harden Oxygen against potential attacks, it is highly recommend you enable this option. It is enabled by default for new installs of Oxygen 2.1 and up.
If you wish to show the invalid signature warning on the front-end of the site, you can enable "Show invalid shortcode signature warnings on the front end" via Oxygen -> Settings -> Security.
Shortcode Signing Explained
Oxygen stores the elements on your page as WordPress shortcodes. At runtime, those shortcodes are compiled into PHP, HTML, CSS, and JavaScript.
If another plugin on your site has a security hole which lets unauthorized users execute shortcodes, they could execute Oxygen's shortcodes. Since Oxygen's shortcodes allow for the execution of PHP, they could run any arbitrary PHP code they want on your site - in other words, change your admin password, steal all your user data, or do literally anything else.
Of course, this attack would require another plugin to be present on your site with a security flaw that allows unauthorized users to execute shortcodes. But since it is likely many of these plugins are floating around, it is highly recommended you enable this option.
The only way to generate a valid signature is to have access to the signing keys, which are unique and randomly generated when you install Oxygen. These are stored in your wp_options table, so only a site admin or an attacker who could otherwise read data from your database could get the keys and sign a shortcode.
Further reading: http://www.pritect.net/blog/wordpress-shortcode-injection-attack-vector
WordPress Multisite
By giving users on a multisite network access to Oxygen, you』re giving them the ability to write PHP code from within Oxygen using elements like Code Block or Easy Posts.
Obviously, this means they could they could execute any arbitrary PHP code - just like if you gave them the privilege to install plugins or edit theme PHP files.