How do I make an appointment without others seeing what it is?
Although Outlook includes a checkbox in its event-creator window called “Private”, according to Microsoft’s support documentation, this feature does not work as one would expect. When using Outlook webmail, other users who can view your calendar will still see the event, but will not be able to see the details, even if their permissions would otherwise allow it. Unfortunately, users who attempt to view the details via some mail clients (such as Outlook for Mac) will be able to see them.
Therefore, it is recommended that truly private events be kept on a separate calendar which you do not share with other users. Then, a “hold” event can be put on your shared calendar so that there are no details being shared, but your unavailability is still apparent.