Office 365 (Outlook 2011) - Subscribe to an external calendar
You can use Outlook 2011 to share your calendar with anyone inside or outside your organization. When you share your calendar with someone inside your organization, they will be able to add it directly to their calendar view in Outlook or Outlook Web App. People outside your organization will be sent a URL that they can use to view your calendar.
But, there is a work around. You can use Outlook on the web to subscribe to an external calendar. Once subscribed via Outlook on the web, the next time you start Outlook 2011, the subscribed calendar will appear within your calendar view.
Use the following instructions under "Outlook on the web" to subscribe to an external calendar: Office 365 (Outlook on the web | Outlook 2016 | Outlook 2013) - Accessing a Published/External/Internet Calendar.