It's only partly open sourced, the premium features are closed (open-core in other words). Also, Open Project is a fork of Redmine. I evaluated both a while ago, and Open Project differs in two ways: fancier design and a broader scope. While redmine is geared towards software development, Open Project has generalized the interface, language and some features to be better suited for any business out of the box. Redmine, on the other hand, has a lot of plugins that is not compatible with open project and can be customized to suit your needs.