Obsidian is not a great replacement for OneNote. I tried switching but there’s a bunch of things like sharing pages (and no, emailing documents doesn’t count), easy syncing between all platforms (Syncthing doesn’t work at all on iOS and was kinda finicky on other things, and git is just not a valid option), it doesn’t do super well when embedding images or PDFs, doesn’t have the same advanced hand writing stuff, and probably some other things that I’m forgetting.

OneNote is basically the only thing besides email that I can’t find a good self hosted alternative. And I’ve been looking trust me. Obsidian is great if all you need is note taking on a desktop, but that’s about where it ends. Or if you want to pay for the subscription and cloud storage, I would imagine it’d work fine.