Ask HN: Is there a OneNote file parser? Or an equivalent opensource note format?

How does OneNote file format work? Is there a parser for it? Looking at the wiki page for onenote: https://en.wikipedia.org/wiki/Microsoft_OneNote

It says .one is an open format but if so where can I find a parser for it or create a .one file programmatically?

Is there an open-office or any such other open source equivalent of note taking software?