A student would like to use Zotero with Onedrive Word Online running on a MAC. Based on some other forum conversations this seems to be doable, but after looking at the documentation I don't see how to get started with this. Can anyone point me in the right direction.
No, Zotero won't work with Word online. Zotero does work with Word 365 & Onedrive, which allows for simultaneous live editing.
Note that you can safely edit a document that contains Zotero citations in Word Online. You just can't add or edit Zotero citations or bibliographies.
How can I get Zotero to work with Word365 and Onedrive? Is there any documentation or instructions on how to do this? Is the answer that I can edit a document that already has Zotero cites in Word 365?
Afaik the only thing needed is to place the document into OneDrive and open it in Word; there's nothing special involved.
If by Word365 you mean the installed desktop version of Word, then yes, you can use Zotero normally to add and edit citations and bibliographies, in both new documents and existing ones.
Word Online does not have the extension system Zotero uses to edit documents, so it's not possible to add Zotero to Word Online. You can edit a document with Zotero citations already in it in Word Online and then go back to working with it in the desktop version of Word with Zotero later, but you cannot make any changes to Zotero citations or bibliographies from within Word Online itself.