A sample of an OPF file.

opf_sample.opf

<?xml version="1.0" encoding="UTF-8"?>
<package xmlns="http://www.idpf.org/2007/opf" dir="ltr" prefix="se: http://standardebooks.org/vocab/1.0" unique-identifier="uid" version="3.0" xml:lang="en-US">    
    <metadata xmlns:dc="http://purl.org/dc/elements/1.1/">
        <!--The DCMES elements-->
        <!--The meta elements-->
        <!--The link elements-->
    </metadata>    
    <manifest>
        <!--The item elements-->
    </manifest>
    <spine toc="ncx">
        <!--The itemref elements-->
    </spine>
    <!--The collection elements-->
    <!--The guide elements-->
    <!--Other elements-->
</package>