There is one efficient way to add the Outlook calendar file to the O365. The experts recommend the SysTools Office 365 Import tool to
import Outlook Calendar to Office 365 without any errors. Go with the below steps for the successful process.
Step 1. Download & select Source and Destination.
Step 2. Choose calendar and Next.
Step 3. Fill in the details of the Source and Destination.
Step 4. Load the users into the tool and click on the Start button.
https://www.emaildoctor.org/blog/outlook-calendar-to-office-365/
