Subpages in OneNote are a good way to visually organize page tabs of notes that belong together. For example, if you have annual and monthly meeting notes, the annual notes can be a main page in your notebook, and you can demote the monthly notes so they become subpages.
Before you can create a subpage, you’ll need at least two pages: One page to be your main page (the parent), and at least one page to be the subpage (the child). If your notebook is empty, first add some pages.
To demote a page to a subpage OneNote Online, do the following:
On the left side of the screen, right-click the page tab that you want to demote and then click Increase Indent.
You can do this up to two times for the same page tab. (Remember, this only works for page tabs shown underneath the first page tab in a section. You cannot make the first page in a section a subpage.)
To change the level of indentation at any time, or to promote a subpage back to a main page, right-click the subpage, and then click Decrease Indent one or two times.