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:
- Back up your database
- Copy all your osCommerce product images into media/import (and make them writable by the webserver)
- VERY IMPORTANT: Change the language for the admin interface to English (United States) - or no products will appear even after a succesful import. You can switch it back afterwards
- Set up your import profile (from System > Import/Export > osCommerce)
- Click on Run Profile on the left, and then the "Check Requirements" button
- If you only have one store/website, the Website Code field will be "base".
Good luck!