By the way, this is covered in the OpenBSD FAQ
. The FAQ should always be the first place you look for answers. I have conventiently linked it from the sticky post at the top of this forum as well.
I'm not trying to be a jerk here, I'm just pointing out that the three "main" BSDs are amazingly well documented. For most questions a quick consultation to the man pages or the website documentation will get your answer, and a whole lot quicker than you normally get a response on a forum. Oh, speaking of man pages I just realized this question is answered in su(1) as well. Read the documentation, that's what it's there for