These simple steps show how to add an Office 365 calendar on Windows.
Install OneCalendar
Download OneCalendar from the store and install it on your device.
Goto OneCalendar Menu
Select the "settings" menu icon on the bottom right.
Select Accounts overview
Select the option "Accounts" in the menu.
Select Add account
Click on the "+" sign to add a new account.
Select Account type
Click on the "Microsoft" account type.
Enter credentials