I would recommend against OCC; for a time, my full-time job was fighting its total lack of reliability and debuggability. It sets a new bar for impossible-to-understand C++ code (a bar I thought boost had set so high it was impassable) and its error reporting is totally useless (every exception is exactly the same: StdFail_NotDone. Good luck figuring out what went wrong)

CGAL's pretty good; they've got an implementation for everything but not all of them are worth using. I just got done reimplementing the RANSAC algorithm in a client's codebase because CGAL's was super slow. But on the whole, it's an amazing feat of open source software.

If you can pony up, though, Parasolid is where it's at. Hands down the most reliable geometry kernel I've ever used, and their support is fantastic.