oScommerce is one very old free ecommerce and thus it has some advantages over OpenCart. In terms of looks, features and robustness, OpenCart is way ahead of oScommerce. Both are open source ecommerce system that can be downloaded free and installed on your own web server that supports PHP and MySQL. Here is a quick review of both which may help you to decide which one is good for you:
oScommerce: Earlier versions of oScommerce were messed up with tricky codes and worst template structure. The current release is much better and now uses DIV’s for template. One thing which is great about oScommerce is the tremendous support it gets from technical users. It has many add-ons available on oscommerce website which are free and add many features to website. The support forum is full of knowledgeable members that help readily. oScommerce needs heavy template customization before it can be used on a live website as default one is still looking pretty outdated. Also one weak point in oScommerce is that the script is itself the template, which means when you modify the design, you automatically change the core files. There is no option to select templates or revert back to default.
OpenCart: OpenCart is something you can use straight after installation as most of the changes can be done from the admin panel itself. Also in terms of looks, the default template is not bad at all. OpenCart features selectable template and template files are separate from core files. The problem arises when you need some feature which is not available in OpenCart. You can find lots of Extensions on OpenCart website but most of them are paid.
Conclusion is if you are a programmer with good knowledge then OpenCart is the clear choice, you can code extensions yourself for special requirements if any. In case you don’t know much about coding but good at designing then oScommerce maybe good for you because you can make it good looking by just changing the design and any feature you ask is available as a free add-on on oScommerce website as contributions from its users.