I think OpenOffice will use UTF-8 by default for its standard
ODT output. So you could try saving in ODT and then converting to ePub using Calibre.
Assuming this isn't enough to get it working with (say) Adobe Digital Editions then you need to embed a Unicode font in the ePub. Practically any Unicode font should support Esperanto. You can test your font by reading the ePub using FBReader (which allows user-specified fonts) and selecting the font from its gear wheel icon and the Styles tag. Once you know it works, I think Sigil can embed a font for you. Any of the GPL licenced fonts at
Unicode typefaces should be suitable.