where to start learning

If you look at the OCC online help under Technical Overview, I
recommend the following order:

- Overview of Object Libraries
- Foundation Classes
- Modeling Data
- Modeling Algorithms
- Visualization
- Application Framework
- Data Exchange

Links to the PDF users guide for these can be found in the
Reference Documentation section of the help. Also, it is a
good idea to check out "My First Application" at:

http://www.opencascade.org/org/gettingstarted/appli/

Finally, the Draw test harness is a valuable tool for
experimentation. So, check out the users' guide under
Development Tools in the Reference Documentation section
of the help.

Good luck

Does http://www.opencascade.org/org/gettingstarted/ make sense ?

Of course, surfing on this site can be of great help.