Turn on external sharing notifications for OneDrive
To help your OneDrive users monitor and control which external users have access to their files, make sure external sharing notifications are turned on. File and folder owners will be emailed when:
Another user invites external users to shared files
An external user accepts an invitation to access their files
If external sharing is enabled in your organization, these notifications are enabled by default.
Office 365 admins can use Search the audit log in the Office 365 Security & Compliance Center to monitor content that is shared externally.
Turn on external sharing notifications
Sign in to the OneDrive admin center as a global or SharePoint admin, and select Notifications in the left pane.
Under Email OneDrive owners when, make sure the following check boxes are selected:
Other users invite additional users to shared files
External users accept invitations to access files
This setting no longer works for the new sharing experience that appears in most places. This setting will be removed.