ODT is OpenDocument text, the native format of LibreOffice Writer. EPUB is a reflowable book split into chapters. pandoc sits between the two, and this page says exactly what happens to an ODT file on the way to becoming an EPUB one.
What runs when an ODT file becomes EPUB
pandoc in two passes, on a machine we rent and watch. Your ODT file is uploaded once, pandoc runs once, the EPUB comes back, and neither file is kept. ODT to EPUB is one of the 3649 pairs that engine was probed on with a real file, which is why it has a page here and why the pairs the probe could not prove do not.
What survives from the ODT into the EPUB
The heading hierarchy. pandoc parses the ODT file into its own document tree and writes that tree out as EPUB, so a level two heading is still a level two heading and not merely bold text.
Tables, rebuilt in EPUB syntax rather than screenshotted. That is the difference between a table a machine can read and a picture of one, and it is the whole reason to send an ODT file through pandoc rather than through a printer.
Lists from the ODT file, nesting included, plus code blocks, block quotes and links, all re-expressed in EPUB.
Images: pandoc extracts them out of the ODT file into a working directory and re-embeds them in the EPUB, so nothing ends up pointing at a file that no longer exists.
What ODT to EPUB costs you
Page layout. ODT carries some and EPUB carries none, so margins, page breaks, headers and footers have nowhere to land and are simply not written.
Where ODT and EPUB files come from
ODT. LibreOffice Writer writes it natively. Like ODS it is ISO/IEC 26300, and it turns up wherever a public body or a policy asked for an open format rather than a vendor one.
If a model is going to read the EPUB
Structure is what survives from the ODT file, and structure is what a model needs. ODT in, EPUB out, with the heading tree intact instead of flattened into one long paragraph.
ODT to EPUB, measured rather than promised
Running ODT to EPUB against the real engine with a real file, pandoc wrote 5,063 bytes of EPUB in 0.1 seconds, machine otherwise idle. That is one ODT file on one day and not an average, which is why the number is given together with the file that produced it.
The ODT to EPUB verdict was reached by a pandoc round trip, meaning the output was read back by pandoc and had to still contain a witness word planted in the source, which proves the content travelled and not merely the container. Which check was used matters, because they do not all prove the same thing, and a status code of 200 proves nothing whatsoever about whether the EPUB file has anything inside it.
The same ODT file, sent somewhere else
Other outputs the probe measured out of an ODT file, so the cost of choosing EPUB can be read against something. Sizes do not compare across engines, because each family was probed with its own ODT witness file.
ODT to PDF: 11,917 bytes in 1.1 seconds, verified by a format signature.
ODT to MD: 277 bytes in 0.1 seconds, verified by a pandoc round trip.
ODT to DOCX: 5,514 bytes in 1.1 seconds, verified by a format signature.
ODT to TXT: 264 bytes in 0.1 seconds, verified by reading the text back.
ODT to HTML: 4,043 bytes in 0.1 seconds, verified by a pandoc round trip.
ODT to RTF: 4,214 bytes in 1.1 seconds, verified by reading the internal type and finding a witness word.
Other ways into EPUB, and what they measured
Among the published routes into EPUB, ODT is the fourth largest output of the 11 measured. The witness files differ, so this ranks the probe run and not your document.
RTF to EPUB: 5,033 bytes in 0.1 seconds.
TXT to EPUB: 4,955 bytes in 0.1 seconds.
MOBI to EPUB: 22,644 bytes in 0.6 seconds.
AZW3 to EPUB: 22,666 bytes in 0.7 seconds.
PDF to EPUB: 26,909 bytes in 0.9 seconds.
What we will not pretend about ODT to EPUB
An ODT file over 25 MB is refused before the upload finishes rather than after it, so you do not wait for a rejection.
An ODT to EPUB run that passes 60 seconds is killed, and the pandoc process is killed with it. A run left behind would sit on one of the machine's two cores until somebody noticed.
32 of the 935 pairs probed in the family that serves ODT to EPUB failed, and this pair is not one of them. They fail for reasons worth knowing: a writer that cannot carry what the document is made of, or output the engine could not read back. They are counted here rather than hidden, because a pair that fails quietly is worse than one that fails loudly.
pandoc never writes the EPUB through a TeX engine here, and it never writes PDF at all: the eleven engines it would need are not on this machine and a TeX distribution weighs several gigabytes. Anyone who wants a PDF out of an ODT file goes through LibreOffice or calibre, both of which genuinely can.
One ODT file at a time, chosen in the browser. There is nothing else to set up and nothing else on offer.
ODT to EPUB: what people ask
What actually converts my ODT file to EPUB?
pandoc does it, in two passes, on a machine we rent and watch. Not a browser trick and not somebody else service: the ODT file is uploaded once, pandoc runs once, the EPUB comes back, and neither file is kept afterwards.
How long does ODT to EPUB take?
On the file the probe used, pandoc took 0.1 seconds and wrote 5,063 bytes of EPUB. That is one real measurement on one real ODT file, not an average and not a promise about yours: a larger ODT takes longer, and past 60 seconds the run is stopped.
What do I lose going from ODT to EPUB?
The one to know about first: Page layout. ODT carries some and EPUB carries none, so margins, page breaks, headers and footers have nowhere to land and are simply not written.
Are the tables in my ODT file still tables in the EPUB?
Yes. pandoc reads the ODT tables into its own document tree and writes them back out in EPUB syntax, so what you get is a table a machine can parse rather than a picture of one.
Is ODT to EPUB free?
There is a free allowance every month, and one ODT to EPUB conversion costs half a credit against it. When the allowance runs out the tool says so and stops, rather than quietly handing you a worse EPUB.