Using Classic Outlook on Windows for personal email, calendar, and contact management
I've found this solution https://support.google.com/calendar/thread/205900609?hl=en&msgid=210862726
Which was also posted here, earlier by Hip Pickles7 (I can't find how to get a link to that comment..)
If you have the outlook app installed on Android, open the app, open settings, click your outlook account, and ensure 'Sync calendar' is turned on.