I had a task of migrating few thousands contacts and some calendars from Apple iCloud to Microsoft Office 365 (Exchange) system.

Importing Calendars was pretty straight forward:

• Export all calendars in .ics format on a Mac via buit-on Calendar (select your calendar > File > Export > Export).
• Open .ics calendar on Windows Machine with MS Outlook 2016 (which already had Office 365 account setup).
  • Go to View > Change View > List.
  • Drag all relevant appointments into Office 365 account calendar (I skipped appointments older than 12 month).

Read more: Migrating contacts and calendar from iCloud to Office 365

This is how I normally create a company-wide Shared Calendar in Office 365:

• Exchange Admin Center > Recipients > Shared
  • Create a new Shared Account
  • Edit account's properties and in Mailbox Delegation  tab add All Users group to Full Access list.

This will give your users permission to add shared calendar to their MS Outlook and Outlook Web App. If you want the shared account (including Calendar) to be added automatically to users' Outlook, add individual users (rather than All Users group) to Mailbox Delegation > Full Access.

If all of your users are on Windows machines that's pretty much it. Unfortunately things are not so simple if they are on MacOS and iOS. I tried to get help from Office 365 support, but received a not very helpful "we don't support Apple products" or something similar... Luckily after some Googling and bunch of trial and error I finally got it working. Here is what I did:

Read more: Office 365 - Accessing Shared Calendar on MacOS and iOS