Hi there, dwleake.
It appears that you're having a problem using Microsoft Office 365 on your MacBook Air. The only way you can autosave a document is by uploading it OneDrive. The problem is, your MacBook Air shuts off unexpectedly which causes you to lose the work you've done. We understand how important it is to have your Mac working as expected, and we're glad to help with this.
How do I turn on AutoSave? - Office Support -- From what we're reading, your documents are stored in OneDrive once you save a new document. Once you start editing, your edits are saved.
Set up iCloud Drive - Apple Support -- Apple uses a similar feature called iCloud Drive. This resource explains more: iCloud Drive FAQ - Apple Support
Add your Desktop and Documents files to iCloud Drive - Apple Support -- This article explains how to start using iCloud Drive on your Mac. You can also access your files on your iOS devices as well as a Windows PC.
If your Mac restarted because of a problem - Apple Support -- The overarching issue is that your MacBook Air shuts down unexpectedly. Even if you haven't seen this particular message, the steps in this article can help.
Feel free to get back to us with any questions or problems you may have.
Take care.