As of June, 2004, SMP (Symmetric MultiProcessor) support has been merged into the main OpenBSD development branch. At this time, the i386, amd64, mvme88k, sparc64, macppc and hppa platforms support SMP. The sgi port supports SMP on the multiprocessor Octane models, other machines are not yet supported. Hopefully other platforms including sparc and alpha will ultimately support SMP.
Please see the appropriate platform page for SMP status for your system.