This article explains how to turn off the "Focussed Inbox" in Office 365.
Outlook for the Web
Step 1: Navigate to the settings menu in the top right-hand corner.
Step 2: Go down to the "Focussed Inbox" switch as shown below and turn it off (white box).
Step 3: You will now see that the "Focussed" and "Other" section have been removed and you have a single Inbox.
Outlook Desktop App
Open Outlook and go to the View tab. From there click "Hide Focussed Inbox". See below.