How to Setup Out of Office Reply on iPhone Outlook App
To turn on Out Of Office notifications via your iPhone Outlook app, follow below:
1. Open Outlook app on iPhone, Click on your Head (upper left)
2. Then, click on the Settings Gear icon (bottom left)
3. Then, Click on your mail account Office 365
4. Then, turn on Automatic Replies
by clicking Off to On

5. Then, you will be able to adjust the reply and time period in following screen