Download and install Dropbox to your PC where you use OneNote and to your Mac. Create Dropbox account on dropbox.com. Then complete sharing in Outline on iPad - long tap on the notebook's cover, choose Sharing > Dropbox > Enter your credentials. Your notebook will get shared to Dropbox.
On your Mac open Outline, click + in the navigation tree next to NOTEBOOKS, choose Open from Mac > Browse for your notebook files in the Dropbox folder on your Mac.
The same with OneNote on your PC.
Illustrated instructions are in the Getting Started notebook.
I hope this info helps. Does it work for you? Please let me know. Looking forward to hearing from you!