Step 1

From your iPhone's home screen, tap Settings

Step 2

Scroll down and tap Mail, Contacts, Calendars

Step 3

Tap Add Account

Step 4

Tap Exchange

Step 5 

Enter in your Office 365 email address, then enter in your password.

You can also give this account a description which is handy if you will be syncing multiple email accounts.

Tap Next

Step 6 

Wait for it to show check marks for everything and then select the services you want to sync