Both OsCommerce and ZenCart are e-commerce and online store management software programs. They both are PHP based and uses MySQL Database as well as both are available for free under GNU.
If there is no experience in the development of Open Source Web Development, OsCommerce is a good option to start with as it is easy to use and very convenient online store solution. It has all basic features needed to build online store with very simple installation process. Compared to ZenCart it is much easier. OsCommerce is the simplest among all other shopping store solutions available so it is most popular for those who want to start for online business. But many functions are not available in it as no upgraded version is introduced in OsCommerce for almost over a year.
ZenCart is more advanced than OsCommerce. It has many advanced functions like configurations can be changed easily within the admin interface, it has in-built engine of sub-pages where information of products can be added easily, allows including the mode of payment in your e-commerce website and many other.
In OsCommerce there is no graphic template system which makes very difficult to modify the design of the website while in ZenCart modifications can be done easily through the template system in the admin interface languages.