To add a new or existing OneNote notebook to a channel, click Add a tab at the top of the channel and select OneNote.
From there, choose an option:
To start from scratch, click Create a new notebook, add a name to the Notebook name field, and click Save.
To add an existing notebook, click Browse notebooks, choose the notebook and section, and click Save.
To add a link to an existing notebook, click Paste a OneNote link, add the address, and click Save.
Once you add the OneNote tab, everyone in the channel will be able to view and make edits (provided they have access to the notebook). Anyone blocked from viewing the notebook can request access right from the tab.