You can view an existing outline in Word Online, but you can’t create and format an outline automatically the way you can in the desktop version of Word.
If you have the desktop version of Word, first use the Open in Word command to open the document in Word.
Then, follow the instructions in Create an outline from scratch.
When you’re done and you've saved the document, you’ll see the outline when you reopen it in Word Online.
Tip: Word Online works seamlessly with Word and other Office desktop programs, and we update it based on input like yours. For the latest feature announcements, visit the Office Web Apps blog.