One of the main advantages of using OneNote instead of paper notebooks is how quickly you can search through all of your notes to find what you’re looking for.
You can use the search feature in OneNote Online, to find specific text on a page or in a specific section. If you'd like to search your whole notebook—or across several notebooks—you can use the OneNote desktop app. For more about how to search across entire notebooks in OneNote 2016, see Search notes in OneNote.
Search a single page
To search a single page, on the page where you want to search, type a word or phrase into the Find on this Page box.
If the word or phrase is on the page, OneNote Online will highlight each instance of it.
Search a section
To search an entire section, click drop-down arrow on the Find on this Page box, and change the search scope to Search this Section.
If the word or phrase is in the section, OneNote Online will highlight each instance of it.