Unfortunatly, osCommerce out of the box isn't a very Search Engine friendly site.  Session parameters get stored in the URL, images don't have alt tags, catagories don't provide any functionality to create unique text, meta tags (including the title tag) are static for the whole site, and more.

Luckly there are many contributions out there that solve many of these problems.  With a little bit of work, you can make osCommerce a search engine friendly site.