Modding phpBB is very similar to OSC, only phpBB is made easy back-end to install templates and you have a better UI in terms of adding content. With OSC all the mods added, much like contributions, are hand-coded. When you install a contribution you modify few lines of code, unless you overwrite existing files with an already modded file (unless you have other contributions installed and don't want to conflict, you would have to manually add them in.). I added a few contributions to the templates, especially Order Editor and Manual Order Maker; all the other cosmetic mods are handcoded in (you can search the lines of code to see where they're entered.).
I just finished a new one a few moments ago, I always start out with a fresh OSC copy and take it on from there. I think it's best to manually do the editing; you have more control over what you do since you're the one physically adding the modification.
Dreamweaver is an amazing tool for editing; that's pretty much all you need besides a good program to search code for exact location of where you want to add an object. That, plus any FTP tool that lets you do live editing.
The few stores I expect on releasing will have all kinds of third-party additions such as a Shout Tag Board, Skype and ICQ integration and built in hit counters (seperate from the contributions you see now.).
If you go to http://www.dynamicdrive.com or http://www.hotscripts.com you can implement any of those in OSC by embading the script code into the existing OSC pages. This gives birth to new options, features, look and feel. Something I expect on becoming big once enough pre-configured stores are released rather than the plain, base OSC cart you can download here.
It would be great to see a better UI for installing mods and editing pages directly through OSC's admin panel, but somehow cross-installing multi-contributions and features always conflict.. Which ends up in 100s of random errors that are visible and some errors that exist but you can't see until someone actually complains "I get an error when I try to.." which could all be avoided if careful testing takes place..
The only way so far I know of, of making the kind of additions and cosmetic change to OSC is by manually editing each file. Cosmetic mods don't affect the store in any way but the look of it. Addition 'Features' always conflicts somehow with an existing module.. if two or more are installed.
Maybe they're going to release an easy UI editor that would prevent conflicting, until that time (which I think will be a very long time) hand-coding and manual editing is your best bet to get the results you want. :)
The cool thing is, OSC is so versitile you can add ANYTHING to it. One thing I love about it and hate about all other stores is that if you want to add a board, a game, a forum, link exchange, directory swaps, image galleries, upload and account systems, etc. It can all be done within minutes if you know how to plug them in.
I found some good tutorials I'm planning on posting on the site, as well as write a few. I am collecting as much information possible at the moment so I can bang out an amazing resource site that would answer all those questions and than some. :)