The OEBPS includes one or more HTML files and an OPF (Open eBook Package File) file in XML format. The HTML files sometimes have a .ODF (Open eBook Document File) extension to indicate that they are not pure HTML files. They are generally expected to be "well formed" meaning that they include the HTML optional closing tags, making them nearly XHTML compliant. See eBook HTML for the acceptable tags.
This is the source file contents that is compiled prior to publication. The compilation process creates files that are not compatible with each other but having the same source makes it easier for publishers. Formats that use this source include MOBI, LIT, and IMP. Of these only MOBI is still supported (now by Amazon) and it is now more likely to use ePub as a source.