Office 365 (Outlook on the web) - How to share your O365 calendar with external collaborators
The following are two solutions on how to share your Office 365 calendar with external collaborators.
Next, click the Gear icon and click Calendar under Your app settings:
Under Shared Calendars, click Publish calendar.:
Select the calendar that you wish to share from the drop down menu.
Next, select the permission levels.
Note: The options are Availability Only, Limited Details, or Full Details. The recipient of this calendar will not be able to modify it in any way. These permission levels only change how much the recipient can view.
Click Save and select the first URL, labeled HTML. Copy and paste it in a email to the recipient.
Note: The recipient will be able to view the calendar in a web browser. Any changes that the client makes to their email will be updated live to the recipient's link.