No. In Word Online you can see watermarks that are already in a document, but you can’t change them or insert new ones.
If you have the Word desktop application, use the Open in Word command to open the document and add or change the watermark there. When you’re done and you save the document, it will continue to be stored where you opened it in Word Online. When you reopen it, you’ll see the watermarks in the doc.
Got a suggestion for this feature?
Vote for an idea, or share a new one in our suggestion box at word.uservoice.com.