If you want to keep working with your notebooks both in PC OneNote and Mac Outline, here is the instruction on how to sync the notebooks.
a) If you have local notebooks in PC OneNote:
1. Open your local notebook in PC OneNote and move it to OneDrive: File > Share > OneDrive Personal > Move notebook;
2. Log in your OneDrive account in Outline > Preferences > Synchronisation;
3. Open your OneDrive notebooks - click "+" at the top of the notebooks list > Open from OneDrive > select the notebook.
4. Feel free to work with your notebook both from Mac Outline and Pc OneNote.
b) If you have local notebooks in Mac Outline:
As Mac OneNote doesn't allow to store the notebooks locally and upload the local notebooks to OneDrive from Mac, the only way of moving the local Mac Outline notebooks to OneDrive is to create a new notebook on OneDrive and to move all the sections from your Outline notebook to this new OneDrive notebook.
1. Create new notebook in OneNote web client;
2. Open this notebook in your Mac Outline with "+" > Open from OneDrive;
3. Drag&drop or cut&paste (with right-click on the section tab > Cut / Paste) all the sections from your local Outline notebook to this new OneDrive notebook;
4. Sync and check in OneNote web client whether the edits were moved to the cloud;