Oscommerce-paypal-wpphosted is a project mainly written in PHP, it's free.
osCommerce implementation of PayPal's iframe based credit card form
Allows using PayPal's hosted credit card processing page in osCommerce as an iframe hosted payment flow.
Download as zip (click Downloads button as above) then extract into the root directory of your osCommerce distribution.
Then go to Modules -> Payments and configure Website Payments Pro - Hosted Solution as per your requirements.