I spent quite a while trying to import an osCommerce store into Magento 1.7.0.2 and wanted to document it somewhere relevant.
The official migration tool seems like the best free option, but it was clearly never completed or properly tested (to be fair, it is marked as Beta).
It's still possible to get it to work, with a lot of tweaks. I was able to import categories, products, orders and customers using the method in my answer below:
Install the extension from http://www.magentocommerce.com/magento-connect/oscommerce-migration-tool.html, but then upgrade it with the most recent version (from 1.5 trunk) in SVN:
svn co http://svn.magentocommerce.com/source/branches/1.5-trunk/app/code/core/Mage/Oscommerce/ Oscommerce
You should then be able to run your import following these instructions:
Good luck!