Dear Aaron Scott (ascott2),
Thanks for posting in Microsoft community. We are glad to help you.
Based on the error message you provided, we understand OneNote for Mac takes up much memory on your device. May I know if you are opening several notebooks with pictures/sheets/inserted files at the same time? Since the local cache contains complete copies of all opennotebooks, close the notebooks you don't need may help you with this issue.
You could also remove OneNote cash manually.
a. Sign out and close all office apps.
b. Go to Finder, find ~/Library/Containers/ (press command + shift + G and enter this path)
c. Find “com.microsoft.onenote.mac” folder, select and move it to trash
d. Restart Mac and sign in
*Note: Please make sure all the notes are synced successfully to the cloud, or you can backup manually your contents before doing this step.
Thank you for your understanding and cooperation! Have a great day.
Sincerely,
Sean | Microsoft Community Moderator