OpenBSD is a fork of the NetBSD project. If you _really_ want to know about it, the OpenBSD founder, Theo De Raadt, has a huge log of e-mails from the NetBSD mailing list discussions leading up to the fork.
http://zeus.theos.com/deraadt/coremail
In any case, all of the *BSDs are originally based on 4.x BSD (Berkeley Systems Distribution), a version of UNIX developed by the University of Califonia at Berkeley.
Each *BSD has it's own set of developers, but a lot of work is shared between the projects (ie. FreeBSD just imported a new initialization system originally made by the NetBSD developers). The projects are usually differentiated by their main goals.
OpenBSD - security and crypto
NetBSD - portability (they support a ton of platforms)
FreeBSD - stability (relatively few supported platforms)
These are generalizations of course. They are all striving to make well rounded distributions. It's just that these are the goals that each projetct is known for.
--
chuckx