how do I set a gp for just one OU?

You just need to create the GPO and link it to the OU you want it to apply to. This assumes, of course, that the OU is at the bottom of a tree. If the OU that has the PCs in it has a child OU with other PCs in it, linking the policy to the first OU would also affect the child OU computers.

what version of windows are you using?

You're probably better off applying this gpo to a security group. Put all the pcs that you want to apply this gpo then filter the gpo to only apply to this group.

But you can right click on the OU and select the option that says "create and link gpo here"
OF course you have to be under group policy for that.

Best practice is to use the filtering so that it wont affect other OUs

ok Server 2008

Take a look at this link. A step by step.

If you need more info let me know, but start by creating a OU for the targeted pcs. Place them in there then follow the link.

Let me know if this helps.

Thanks,

Vic

Ummm what link?

http://msdn.microsoft.com/en-us/library/windows/desktop/aa373513(v=vs.85).aspx

just for some knowledge short article on Scope of GPO management