If your organization provides business users with the possibility to manage sites themselves by assigning them site collection administrator rights, you may want to know if they add anyone else as site collection administrator. While you could run a script regularly to retrieve a list of all site collection administrators on all sites and then compare this data with previous information, there is also an alternative that you can use. Office 365 provides you with the option to get alerted when someone gets added as a site collection administrator.
First, open the Office 365 Security & Compliance portal. Select Alerts and ‘Manage alerts’, then click on ‘Add an alert’ in the right content pane
Setting up the alert is fairly easy. Start by specifying a name and optionally a description. The most important setting is “Send this alert when…” where you specify which activities you want to get alerted on. Select ‘Added site collection admin’ here. Sadly, there is no way to monitor removal of site collection admins.
Lastly, specify who should be alerted, and save your alert.
Please note the following: “It can take up to 30 minutes or up to 24 hours after an event occurs for the corresponding audit log entry to be displayed in the search results.” (Search the audit log in the Office 365 Security & Compliance Center). This means that notifications are not immediate, but may take some time until the corresponding event appears in the audit logs and the alert gets triggered.