When oscommerce is installed, it is installed into a directory named catalog, unless the installer tells it to do otherwise. This works fine but is not the ideal situation for getting the most from your shop.
The problem is exasperated by some hosts that use html redirects to point the domain to the correct directory. This is not good. Many search engines do not like this type of redirect and will penalize the site for it. In some cases, the site can actually be banned.
So for a more professional appearance and better search engine placement, do your site a favor and have it moved to the root.