In OneNote Online, subpages are a good way to visually organize pages 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 pages below it so they appear as related 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 first subpage immediately underneath it (the child). If your notebook is empty, first add some pages.
To demote any page to a subpage in OneNote Online, do the following:
Right-click (PC) or Control-click (Mac) the name of the page that you want to demote to a subpage, and then click Increase Indent.
You can do this up to two times for the same page. (Remember, this only works for page names shown underneath the first page 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 (PC) or Control-click (Mac) the name of the subpage, and then click Decrease Indent one or two times, as needed.