Where does OneNote for Mac store its notebook files? "Create a new notebook" with OneNote for Mac is not possible in offline mode. :(
Editing and updating your notebook, however, works flawlessly in offline mode.

My question is: where does OneNote for Mac store its Notebook files (for offline work)?  

Also, I would love to find a workaround to use OneNote in offline mode and to block sync in my firewall.
OneNote stores things in an application container in your user's Library folder. Open Finder and use the go to folder function and paste 

~/Library/Containers/com.microsoft.onenote.mac/Data/Library/Application Support/Microsoft User Data/OneNote/


The current version has an offline data file in a 15.0 subfolder. You can snoop on the program when it's running to see what files and ports are open.

sudo lsof | grep OneNote | grep -v Applications