What ODT does well
The ODT format is a ZIP archive containing structured XML — similar architecture to .docx, but cleaner. The XML is more readable, the styling model is more orthogonal, and there's less of the run-fragmentation noise that plagues Word documents. Conversion to Markdown is correspondingly cleaner.
What's preserved
All standard structural and styling elements: headings (Heading 1 through 10 → # through ##########), ordered/unordered lists with deep nesting, tables (better support for ODT than for .docx because ODT's table model is more uniform), bold/italic/underline/strikethrough, hyperlinks, footnotes, embedded images, frames containing text. Styles defined in the document's style section are mapped to semantic Markdown where possible.
Where ODT comes from
French government workflows (ODF mandated for some uses), German federal IT (longstanding ODF policy), Brazilian government, schools and universities worldwide, Linux home users, Apache OpenOffice and LibreOffice users globally. If you receive a .odt and need it in Markdown, this is the path.