Outlook Desktop
You will need to get an ICS link to subscribe to the calendar. How to get this varies based on the calendar you wish to subscribe to.
Go to your Outlook app.
Select the Calendar View.
Click the Add Calendar button and select From Internet.
Paste the calendar link you found earlier.
Press OK.
You should now see the Calendar under Other Calendars.
Outlook for the Web
Go to the web version of outlook.
Select Calendar View
Click Add Calendar
From the list on the left, select Subscribe from the web.
Paste the link of the calendar you would like to add.
Click import.
The calendar should now show under My Calendars.