This conversion is usually related to attempt to convert digital book to audio book, i.e. conversion of ePUB ebooks to Ogg Vorbis Audio.
The most reliably way to do this, would be to use some text-to-speech converter, such as Balabolka, that will generate audio file (.ogg) from text source (.epub), which should be enough for the purpose of epub to ogg conversion. The audio typically uses the voices provided by Microsoft, but some programs have their own unique ones.