If you frequently work in multiple notebooks and want to remove clutter from your list of open notebooks, you can close any notebooks that you’re no longer actively working in.
Do the following:
On the left side of the OneNote app window, under the ribbon, click the name of the current notebook.
In the list of open notebooks that appears, move the mouse pointer over the name of the notebook that you want to close, click the button just to the right of it, and then click Close.
Tip: If necessary, you can reopen a closed notebook again at any time.
Note: Closing a notebook does not automatically delete it. For more information, see Delete a notebook in OneNote 2016 for Mac.