Also here:
Word 2007 & Later Key Data File Locations (pinned in this forum)
Although that is a Windows resource, it gives the file name: Custom.dic which should be the same on the Mac. (I see from Moonshine's link a different name of "Custom Dictionary.") You may or may not need to change the name with Office 365.

See also: Mastering the Spelling Checker by Suzanne Barnhill, MVP


The Custom dictionary is a text file sorted in alphabetical order, one word per line. It can be edited in a text editor as well as in Word.