Why would I choose OpenBSD over FreeBSD?
That depends on what your use case is and what your values are. It may well be that FreeBSD is a better fit for you.
I can only speak for the reasons why I chose OpenBSD over FreeBSD, which are portability, simplicity, correctness and freedom. All of these things help to limit the number of bugs by making the codebase easy to understand, which also makes it easy to modify when I need to.
Portability ensures a clean separation of machine-dependent and machine-independent code, while also enhancing correctness by exposing bugs that only trigger on some platforms (endianness bugs being a common example). Simplicity makes it easy to get up to speed on how a subsystem works by reducing the amount of things to learn. Correctness, in the case of OpenBSD, means that changes don't get committed until they are well understood, well tested and well documented, which again reduces the likelihood of bugs, and existing code is periodically audited to the same end.
The last point, freedom, has two sides to it. On the one hand, OpenBSD's rejection of binary drivers (in contrast with FreeBSD) goes back to what I was saying about correctness; changes don't get committed until they are well understood, and it's impossible to understand a binary blob. On the other, rejection of viral licences like the GPL keeps OpenBSD suitable for use in commercial applications, which isn't so important for my use case, but is something I value just the same.
OpenBSD's perceived emphasis on security is really just a consequence of those four points. Contrary to popular belief, security isn't something you can attain simply by throwing more money at a problem or hiring more developers. Real security mandates that your developers have a disciplined approach to the software they write and that they understand it well, which is exactly what OpenBSD provides.