Hi Madasamy,
<br>
The osz is simply a zip file which contains a osj-file (in json-format) which describes the saved service and contains text-based slides (songs, bible, custom), and if any media (video, audio, images) or presentations (ppt, pdf) files is included they are saved in the zip-file as well.
I don't know of any official spec for the osz-file, so I hope this helps a bit. You should probably just create a file in OpenLP, unzip it and look at the content.