Step 1: Open the settings of the iPhone
Step 2: Scroll down and tap “Accounts & Passwords” (about half way down the page)
Step 3: Tap the office 365 account you wish to enable out of office on.
This is the 1st item down in the list on my test phone.
Step 4: Scroll to the bottom of page and click on Automatic reply
Step 5: Setup your out of office messages
You can set up 2 different messages one for internal staff the other for people outside the organisation
Click save. Your out of office is now on.