You may have just migrated from your old email provider to O365, but then discovered that your calendar didn't come along for the ride. Using this method, you can migrate your calendar data into your new O365 account.
This is best achieved using 2 Microsoft Outlook profiles, one connecting to your existing calendar (your old account) and one connecting to your new O365 Account.
Open your old mailbox profile in outlook.
Click file, open & export, Import/Export
Pick export to a file
Choose Outlook data file
Highlight Calendar, and tick Include subfolders
Give it a file to export to.
Enter a really difficult password that you won't forget.
And enter a different really difficult password you won't forget
Let it export
Now switch to your new profile
Click file, open & export, Import/Export again.
Pick Import from another program or file
Choose your file, and click "open"
Enter the passwords you didn't forget
Select the calendar
Tick include subfolders
Select Import items into the same folder in:
Import the calendar
Check your calendar looks right.
You can now delete the import .pst file, and depending on your configuration you might also want to clear the old calendar to avoid double-reminders. Do this in your old outlook profile, and only when you are ready and sure your migration was completely successful, obviously.
Delete all items from the primary calendar
Although you can’t delete the primary Outlook calendar, you can delete all the items in that calendar.
Warning: The following steps will delete all items in your calendar, including holidays that were automatically added. See Add holidays to the calendar to make the default Outlook holidays that were deleted available again. All other calendar items are permanently deleted.
In Calendar, click View.
In the Current View group, click Change View, and then click List.
Click in the item list, and then press Ctrl+A to select all of the items in the list.