Changing the .ics file association to always open in Outlook
If you only use the Outlook calendar on your Mac, you can ensure that .ics files always open in Outlook as opposed to the Mac OS calendar app.
If you use Outlook for work but the MacOS Calendar for personal use, you may want to keep the .ics association with the Calendar app and import individual events to your work/Outlook calendar when needed. See Importing an .ics file into Outlook on a Mac for instructions.
Instructions
Use Finder to locate an .ics file
Right-click (touchpad: two-finger tap) on the file to display the file options menu
Select the Get Info option
Select/Change the Open with: option to Microsoft Outlook and click Change all…
Click Continue to confirm the change when prompted