While the page list in Microsoft OneNote always appears full-screen on iPhone, you can set your preference for its position on either side of the screen on iPad.
Tap the ≡ icon in the upper left of the app.
In the sidebar that opens, tap the gear icon in the upper right.
On the Settings page, tap Interface, and then turn on the Flip Page List Location option.
This feature is currently not available when using Microsoft OneNote on iPhone.