I'm not sure, I've never tried to install xp oem on a second system(I think doing so is agaist what's allowed in the Eula anyway). I've heard people say that oem can only be for one system, and the retail can be moved from computer to computer(as long as it's only on one pc at a time). I'm not sure if it's actually "tied" to the motherboard. If it was, then people couldn't get replacement mb's if they wanted to upgrade, since it'd be different.
Oh, wait, I did replace the mb in my dell pc, I guess dell xp is oem. It worked fine when I replaced the board and repair installed, so I guess it's not tied(?), but I haven't tried doing a clean install on the new board yet to know for sure.