one: the paypal_standard that OPC supports does IPN notifications, it is just not called "Paypal IPN" it is the best one created by the OSC team and included in the RC 2.2a http://www.oscommerce.com/community/contributions,5655/category,1/search,paypal+standard
two: for those that want to use paypal express with one page checkout, there had to be some changes to the express module since it returns the user to checkout_confirmation.php from the regular 4 step checkout. If you use it then install the express module included with the OPC distribution.