We can setup office 365 email account on iPhone using below steps.
1) Tap on Settings.
2) Tap Mail. Then tap Add Account.
3) Tap Add Account.
4) Tap Exchange.
5) Enter your ofice 365 email address and password. Then tap Next.
Enter your office 365 email address as email@example.com (email address). Then type your current office 365 email password.
Type a description for the account (optional).
6) Enter the server as outlook.office365.com and your username as firstname.lastname@example.org (your email address). Then tap Next.
Enter outlook.office365.com as the server name.
Enter your username as email@example.com (your email address). Then type in your current email password.
NOTE: Leave the Domain field blank.
7) Select the types of account information you want to sync on this device. Then tap Save.
Set the slider to green next to each account item you want to be able to view on this device. Then tap Save.
8) That's it. Once you start seeing your emails appear in the email app, you're all done.