Obsidian is not just an alternative, it is better.
Getting rid of all the formatting bullshit makes notetaking better. Honestly, I consider this to be the killer feature of Obsidian -- no styles. No fonts. No font sizes. No weird/unpredictable line breaks or fights with bullets. No jimmying about images trying to get them in the right places. OneNote needs a plaintext mode to even hope to compete.
The linking is nice. I am very skeptical that the knowledge graph is useful, but I won't be mad at people who like it.
Once you're used to the mathtex syntax, it's a fine way to do formula entry. And with the right extension, the way it handles tables is just fine.
The only thing OneNote does that Obsidian doesn't is nested notes. I really wish Obsidian let you define an "index" note for a folder that would let you mimic the nested note feature of OneNote. And that it would let you manually re-order notes to be in whatever order you want (maybe achieved by a TOC on the index note or some such). Maybe there's an extension that does that? OP seems to want the same functionality. I work around it by just making an "!TOC" file that sits in roots rather than relying on actual file hierarchy.