To enable automatic replies (Out of Office) in the online version of Outlook (Outlook on the web), follow these steps:
Sign in to your Outlook account at https://outlook.office.com
Click the gear icon (⚙️) in the top-right corner to open Settings.
At the bottom of the Settings pane, click "View all Outlook settings."
In the left sidebar, go to:
Mail > Automatic replies
Toggle "Turn on automatic replies" to On.
(Optional) Check "Send replies only during a time period" to schedule when replies start and stop.
Type your automatic reply message in the text box:
You can set a different message for internal (within your organisation) and external (outside your organisation) contacts.
(Optional) Check:
"Send replies outside your organisation"
Choose whether to send to all external senders or only your contacts.
Click Save.
Once this is set, Outlook will automatically send your specified message during the time range or until you turn it off.