SMP (Symmetric MultiProcessor) support is not yet in OpenBSD, but there exists a project, started in February 2000, to bring said support to at least the i386 and sparc platforms.
We have created a src/sys branch in CVS, called SMP where the development is happening. In order to check it out, do:
$ cvs checkout -rSMP src/sys
There also exists a mailinglist in the openbsd.org domain; smp@, for the purpose of discussing the SMP support. Look here for a description on how to subscribe to our mailinglists.
We plan to put up links good documents here whenever we find them:
www@openbsd.org