How do you export an Outlook calendar to a .ics file for use with other calendar services? doesn’t make it easy, but the correct link for this as of July 2018 is this:

Click on Options > Calendar > Shared Calendars > Calendar Publishing, select the right calendar, and then Create a link for sharing. The .ics format will appear. Calendar to .ICS file


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.