There isn't a direct Save as iCalendar option in Outlook for Mac, you can still create a .ics file (iCalendar) on your Mac by using the following methods.
1. Forward as iCalendar Method:
a) Select the event for which a .ics file needs to be created.
b) In the event box, click the Forward button.
c) Click the little down arrow next to Forward in the Forward window, then choose Forward as iCalendar.
d) An email message with the event attached as a.ics file will be created as a result. After that, you can store the attachment to the location of your choice.
1. Forward as iCalendar Method:
a) Select the event for which a .ics file needs to be created.
b) In the event box, click the Forward button.
c) Click the little down arrow next to Forward in the Forward window, then choose Forward as iCalendar.
d) An email message with the event attached as a.ics file will be created as a result. After that, you can store the attachment to the location of your choice.