Based on my understanding, there are only one OU that containing all the users. But you want to deploy different map drives to users in individual countries, right?
If i misunderstand you, feel free to let me know.
Since the map drive policy have the Item-Level targeting setting:
You can target the objects with groups or sites, ect.
In this way, we can deploy different map drive GPOs to different users even there are in the same OU.