OsCommerce is a nightmare for templates, but basically you get your template package and you replace everything the package gives you with your OsC Setup. There is no templating system, its just replacing files.
One of the best E-Commerce solutions that I deploy with most of my clients that require a shopping cart is CubeCart. The flexibility and the ease of styling it to your needs is amazing. It uses smarty templates and is by far the easiest over OsCommerce and ZenCart.