PythonOCC is a project that aims at binding the whole range of OpenCasCADe functions into a python module. This is a different approach than FreeCAD, where only certain components of OpenCasCade are used, resulting in a much simpler structure.
PythonOCC, on the other hand, since it provides you access to all of OCC classes and functions, is very complex, but also very powerful. It is therefore a very fine addition to FreeCAD. When you are limited by FreeCAD's available OCC functionality in your python scripts, it's time to load pythonOCC.
Currently in the Part module we have the methods: Part.__toPythonOCC__() and Part.__fromPythonOCC__() to exchange TopoDS_Shape entities to/from pythonOCC. This allows to use the full power of OCC in python (using pythonocc) and then put the resulting shapes back to FreeCAD.