it all depends on your logic and needs
there could be logon hours
IT management
business logical groupings
The point of OU is a way to group and manage your structure with a logic that you define.
My OU structure is location based (city) then by department
It could have been the other way around, but my company has location based resources (servers printers, etc) so location then department was determined to be most efficient.
with 20000 users, I'd go with the forest with 2 domain (or more) model as mentioned before
