You can have the following payment modules installed on your OsCommerce.

a) If you want to have the PayPal, Authorize.net or 2CheckOut.com modules installed on your OsCommerce you could refer to the following tutorial:

http://www.siteground.com/tutorials/oscommerce/oscommerce_payment_module.htm

b) If you want to have the WorldPay module installed on your OsCommerce  you should refer to the following tutorial:

http://www.siteground.com/tutorials/oscommerce/oscommerce_worldpay_module.htm