How do you export an Outlook calendar to a .ics file for use with other calendar services? Outlook.com doesn’t make it easy, but the correct link for this as of July 2018 is this: https://outlook.live.com/owa/?path=/options/calendarpublishing
Click on Options > Calendar > Shared Calendars > Calendar Publishing, select the right calendar, and then Create a link for sharing. The .ics format will appear.
Don’t forget that it’s also possible to download ICS files from iCloud through the web by replacing the publicly available webcal:// address with a standard https:// and then by adding an .ics at the end of the resulting downloaded file.