A few questions first:
Are the "Roaming users" on the same LAN as your DHCP server?
Do the other computers on your LAN have static IP addresses?
Are the "Roaming users" using laptops or will they be logging into different PC's?
DHCP willnot cross a router by default. If you must cross a router you will need a DHCP relay agent on the side of the router that does not have a DHCP server.
If you have a computer on a LAN set to recieve a DHCP address it will automatically look for the first DHCP server that it can communicate with.
DHCP is not capable of discerning between OU's and will give an address to any DHCP enabled PC that resides on your LAN.
Hope this helps.