Below is Erik's reply to a similar question on this forum a few days ago:
"There is no format painter for OneNote. The closest workaround might be to
copy/paste the text whose format you want to re-use then type over it with
the replacement text. You're essentially placing the formatting *before*
you start to type, which is not normally the way people work.
You might also have some luck with the Paste Formatting menu that appears
when you paste text, but I've found that to be inconsistent in how it
applies the "destination formatting". "
