One of the main advantages of using Microsoft OneNote instead of paper notebooks is how quickly you can search through all of your notes to find what you’re looking for.
On any page of any notebook, tap the magnifying glass icon in the upper left corner of the app.
Type a keyword or phrase into the search box at the top of the screen, and then tap Search.
Note: If you're using OneNote on an iPad, you can tap All Notebooks to select whether OneNote should search within the current notebook or only within the current section.
In the list of results, tap the page on which your search text is found and OneNote will navigate to that page.