Writer2xhtml does a good job, and it has an epub export option that will make the epub package into the bargain. You will still need to tweak the epub with Sigil, however (IMHO).
Another OpenOffice / LibreOffice add-in is e-book Luke's
writer2epub, which takes a slightly different approach to styling, but also produces a reasonable, valid epub. For production quality, you will also have to tweak it in Sigil. There's a forum for writer2epub here on MR.