We are happy to announce that this feature is available to all O365 subscription users with 15.35 (170610) or later build.
Please refer to our post for details:
214 votesMr A BLANES commented
The ability to mark items as 'Done' (marks an email as complete, moves it to a folder, and marks it as read) is essential to my daily workflow.
Support for Google Calendars is now fully available as of 16.13.513+ builds of Outlook for Mac. Note that this requires Office 365 subscription and re-adding your Google account again via Tools > Accounts (existing IMAP accounts are left as-is).
Please contact us using Help > Contact Support if you have any questions or issues.