Open the Settings app on your iPhone
Tap Apps
Tap Mail
Tap Mail Accounts
Tap Add Account
Tap Outlook.com
Sign in with your Outlook account
Turn on Calendar
Tap Save
Open the Calendar app to confirm the Outlook calendar appears
If needed, go to Settings
Tap Calendar
Tap Accounts
Select your Outlook account
Make sure Calendars is enabled
If you use the Outlook app, install it from the App Store
Sign in to the Outlook app
Allow calendar access when prompted
Check the Calendar tab in the Outlook app to view synced events