I'm sad you didn't come to me first for this.
I've never used OneNote before, so I don't really know what the equivalent is, but if I were going to take notes for a class or whatnot I'd probably use
Tomboy. Google seems to agree that this is the closest equivalent, so apart from running OneNote in wine that may be your best choice.
You probably already have either Tomboy or Gnote (which is a clone with a slightly different implementation -- the differences are highly technical and not all that interesting to anyone who isn't a developer, including me). Couldn't tell you how to find it in Unity, though -- that UI is simply awful.
So, uh, good luck with that.