This is complicated to answer, several papers exist which explain the security features implemented in OpenBSD.
The developer Matthieu Herrb wrote one such paper for h2k9, it's listed at
http://www.openbsd.org/papers/.
A Wikipedia article also exists, but, make sure you follow the references, as you have quite a lot of reading to do.
http://en.wikipedia.org/wiki/OpenBSD_security_features