Not in Word Online, though you’ll see equations that are already in your document. If you have the desktop version of Word, that’s where you can add or edit mathematical equations.
To open your document in the desktop version of Word, click Edit Document > Edit in Word, or click Open in Word.
When you save the document and reopen it in Word Online, you’ll see the changes you made.
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.