osCommerce is a complete, standalone ecommerce solution. It does not have a "Add to cart" link you can add, and it needs to manage the product database. You have at least two choices:

1) Install osCommerce in a directory such as public_html/eStore and make a link to it (Go to <a href="/eStore/index.php">My Store</a> for great deals!) from your Home Page in public_html/index.html. You will need to enter all your products into osCommerce, but that one-time chore will make it much easier in the future. You can tweak osCommerce's stylesheet.css file to get colors and fonts similar to your existing pages', but osCommerce has its own page layout that you may or may not like. If you already installed osCommerce into your root directory (/), don't worry -- it's PHP and won't have any effect on your HTML code.

2) Keep your existing HTML code and product information, and use PayPal's drop-in "Add to cart" code. This is a grafted-on shopping cart that takes your customers to the PayPal site to process the shopping cart, and of course to make payment through PayPal. The upside is that it's a relatively minor change to your existing HTML code. The downside is that it looks grafted on, goes offsite to do anything, and unless you have a rather static set of products, it's a pain to change that stuff in HTML code rather than in a database with an administrative interface such as osCommerce's.