For office 365 calendars we recommend trying the following:
Please first decouple your office 365 email address from all the other calendar related accounts where it's connected to (phone settings > accounts). You'll then need to go to phone settings > accounts > add new account > Exchange, as you'll want to add it as an Exchange Activesync account (you can sync email/contacts/calendars all within this same account type, please also make sure the original calendar app and email app that came with your phone are not disabled).
Next you will need to sign in to your office 365 account. Please see our help page here: https://digibites.zendesk.com/hc/en-us/articles/206762509
This is for regular Outlook.com/Exchange accounts, so for office 365 accounts; at the part where it asks for manual set up (inside section 4, part C) and where you have to fill in email/domain/server etc. You'll need to refer to the below info instead:
Email Make sure your full email address is in this box, for example, firstname.lastname@example.org.
Domain\Username Type your full email address in this box, for example, email@example.com. If Domain and Username are separate text boxes in your version of Android, leave the Domain box empty, and type your full email address in the Username box.
Password Use the password that you use to access your account.
Exchange Server The server name for Office 365 for business is outlook.office365.com
Make sure the Use secure connections (SSL) box is checked.