Yes, there is one manual method that you can use to import Outlook Calendar to Office 365 efficiently. However, this method is only applicable to a smaller number of users. By using the below steps you can complete the process.

Step 1. Login to the Office 365 account.

Step 2. Move to Calendar and then click on the from file option.

Step 3. Browse the ICS file from the saved location.

Step 4. Click on the Import option.