maybe it is your question .. or lack there of? =)
i've worked pretty in depth with OSCommerce, .. like any app your gonna hack .. just spend a LOT of time with the back-end in something like phpmyadmin and analyze the data structure so you know what's going on.
OSCommerce is some pretty in-depth scripts, with a heirarchy that will make even a very seasoned programmer's gears turn..
With that said, you will have to spend some time understanding how they have layed out the files.
I wouldn't expect to get it right the first time, so with this in mind, just make a scratch project .. tweek some code, then see what it physically changed on the pages.
Now I may not have given you anything that you were really looking for because I'm not sure what your question really is.
A pretty good resource is the OSCommerce forums. There are a lot of people on there who are very familiar with the software .. that is if you can't find your answers through searches.
Good Luck with your project. It sounds cool.
