You can open Onenote files directly on your Mac, with no need to go through Evernote on a PC, etc. Here is how. With a Onenote file open in a browser save the whole page as an html file (not webarchive). Do it on the desktop for the sake of clarity if you need to. Your html file will have its own folder alongside of it. Open this folder and scroll until you find a file called onenoteframe.html.
Don't open it with a browser. Do not just click on this file. Open it with TextEdit or with Evernote right on your Mac. You can open this file with any editor that will open html. Right click to "Open with") You will probably have to erase a little unwanted info from the Onenote website, but you will have your complete formated file. Evernote renders many unwanted icon images from the webpage that need to be deleted but produces a better copy all in all. - Max