Open the Settings app on your iPhone
Tap Calendar
Tap Accounts
Tap Add Account
Select Outlook.com or Microsoft Exchange
Sign in with your Outlook account
Turn on Calendar sync
Open the Calendar app to check that Outlook events appear
If needed, go to Settings
Tap Calendar
Tap Accounts
Tap your Outlook account
Make sure Calendar is enabled
If you use the Outlook app, open the app and sign in
Allow calendar permissions when prompted
In the Outlook app, tap your profile icon
Tap Settings
Tap your account
Turn on Sync Calendars
Refresh the Calendar app if events do not appear right away
Restart the iPhone if syncing is still not working