Unlike in other Microsoft Office applications, there's no Save button in OneNote. That's because you never need to manually save your work in OneNote.
OneNote automatically and continually remembers and saves everything you do—including typing, editing, formatting, organizing, searching, and sharing your notes.
You can safely exit OneNote at any time without first closing your notebooks or manually saving any of your work. OneNote saves and retrieves everything automatically.
Try it out for yourself. Go to any page in your notebook, type some text, and then immediately exit OneNote. Start it up again and you'll see the last page you were on—complete with the last text you typed.