Follow these steps to add a calendar that's already setup to be shared with you in Office 365.
If you've not already setup calendar sharing, please take a look at the following link:
- Open the Apple Calendar app.
- Go to File > Preferences.
- Select the Accounts tab.
- Select your Exchange (Office 365) account.
- Select Delegation in the right hand side of the window.
- Click on + at the bottom of the list of "Accounts I can access:"
- Start to type the email address or user's name you wish to add, and when you see it appear in the list, click on it to add the calendar to the list.
- Make sure you tick the box in the Show column in order to display the calendar.
- Repeat steps 6 to 8 for each calendar you want to add.
- Close the Accounts / Preferences window once you're done.
- You should see the calendars appear in the Calendars list (you can display the list by clicking Calendars in the top left of the window).