I switched from OneNote to Logseq. Its feature set is pretty much completely different, but in the end I realized it's fine with me and resulted in my notes being more useful.

The main downside that I see now is that it's kind of slow - much faster than the Electron version of OneNote was last time I used it, but slower than old native OneNote app or Obsidian. Otherwise its main differences from Obsidian are that in Obsidian the basic building unit is a page, whereas in Logseq it's a paragraph (and, usually, its sub-paragraphs - it's an outliner), which Obsidian can only do with plug-ins and not as seamlessly, and that with Obsidian you pretty much need to use community plug-ins, whereas with Logseq a lot of the functionality is built-in.

It's open-source and uses markdown, not completely standard, but close enough for the files to be entirely usable if Logseq ever dies. Its community is smaller than with Obsidian, which is a downside, but it's not exactly obscure either.

Really probably the most important thing about Obsidian and Logseq is to read an article or watch a video about how automatic backlinking works. It's especially useful for something like Zettelkasten, but it also works for more "normal" approaches as well as concepts like Getting Things Done.

Both are OK tools and are similar in many ways, but they're quite different from OneNote. Downside of both is that synchronization between devices sometimes creates issues unless you use their paid service.