This is a contribution I found on the phplist forums.
Basically, it changes the newsletter options built-in to OsCommerce, with the PHPList database instead.
NOTE: I did not write this contribution. I have simply put it on OsCommerce for everyone to use.
I am going to change this so that on my Contact Us page, it gives the option to subscribe to the newsletter also.
Installation instructions are enclosed.
View the support thread here: http://forums.oscommerce.com/index.php?showtopic=191317