This article details how to turn off the new Focused Inbox feature in Outlook on the Web.
In order to turn off Focused Inbox first navigate to your Outlook Inbox.
1. Next to where it says Focused Other in the upper left, select Filter:
2. Select Show Focused Inbox at the bottom of the drop-down menu, it should have a check-mark next to it:
3. You should no longer see a Focused Inbox, only a Inbox header with your emails beneath it: