Traditionally, OpenBSD has made "Portable" versions of their code available for integration on other systems - for example:

http://www.openssh.com/portable.html

Basically, that's the version that gets those sorts of patches.

I'd imagine if this reaches the point where people want to put the OpenBSD fork of OpenSSL on other platforms, a similar approach would be used.

It's been pretty successful for OpenSSH:

http://www.openssh.com/usage/graphs.html

Nearly every operating system distribution maintains it's own fork of most system utilities and libraries with varying levels of divergence from upstream. A piece of recent news around here was Python 2.7 being maintained by RedHat for several years beyond upstream support. Some of the differences make it upstream, some of them don't. Doubtful OpenBSD is trying to usurp the current OpenSSL team, but if they do good quality work, it might be very influential a few years down the road.

>Is it guaranteed that this fork will be used in OpenBSD?

It already is used in OpenBSD. The changes are being made in the OpenBSD source tree.

As a Linux fan who has been hearing good things about FreeBSD, will this work out of the box on FreeBSD?

I think it will require a bit of tweaking, but I would expect the other BSDs to adopt it once it is reasonably complete.