More specifically, if I intentionally install all kinds of viruses and malwares on a Windows installation running inside VirtualBox on Mac OS X, can I cause any damage to Mac OS or even to the computer itself (I'm thinking HD partitions, etc.)?
Most of the question is irrelevant: "is the ... 100% protected from security issues ...?" No.
Add in your choice of: computer, business, person. Still No.
More specifically to your question: Since the VM host is not perfectly it is vulnerable to both attacks from within the guest as well as software issues in the guest. If you tickle the right bug in the VM container (you're using VirtualBox) it might crash or cause other problems in the host (the computer running VirtualBox).
Data from the virtual machine is still stored in a file on the host machine, and therefore can never be 100% secure....
The actual host->guest relationship is very secure, but how do you intend to get a virus onto the guest machine without either plugging a device into the physical host, or connecting them to the same network? That's your real point of failure, not virtualised hardware.