It is the international standard for a word-processor document, and Word opens it too.
Convert an .odt file to Markdown
An `.odt` is a zip of XML — `content.xml` for the words, `styles.xml` for how they look, a `Pictures` folder — and it is OpenDocument Text, an ISO standard rather than one company’s format. LibreOffice and OpenOffice write it by default, and Google Docs hands one back under File, Download.
LibreOffice is the obvious answer and it is free on every desktop system. Microsoft Word has opened and saved `.odt` since 2007, and so has Word on the web; Google Docs imports one through File, Open. Apple Pages opens one too, though it will want to save it back as something else.
If all you need is to read it, the zip is open to you: unpack it and `content.xml` is the document, tags and all.
Round-tripping through Word. An `.odt` opened in Word and saved again keeps its words and loses some of its styling, because the two applications do not agree on what every style means — which is a problem only if somebody else is going to open it in LibreOffice afterwards.
Fonts, as with every document format. A file that names a font your machine does not have is laid out in whatever the reader substitutes, and a page count that mattered stops being the same page count.
This is the format that converts most faithfully of the lot, because it states what things are rather than how they look: a heading knows its own level, a list knows its nesting, a table is a table and a footnote is a footnote. Converting one gives you Markdown with all of that, the pictures carried inside the file.