Marking a meeting or appointment private can hide some of the meeting details from others in your organization.
If you share and grant Read permissions to your calendar to another user, they will not be able to see the details of your private appointments. However, if you share and grant Delegate with access to view private items permissions, those delegates will be able to view details of your private appointments. By default, delegates do not have access to private items, but you can change this option when assigning Delegate permissions.
New Outlook
In the calendar, double-click to edit the event that you organized and want to make private.
On the Event tab, select Private.
If you don't see Private, on the right end of the toolbar, select More options > Private.
Save the appointment or send the meeting update. The details will be visible to only you and the meeting attendees. In calendar view, a lock symbol appears on the event to indicate it is private.
Classic Outlook
In the calendar, select the meeting or appointment you want to make private.
In the Tags group, select Private. In calendar view, a lock symbol will appear on the event to indicate it is private.