When you open the ADUC MMC with the advanced view, there will be pre-setup folders. But not all of these are OUs. Most of these are containers:

The only default OU in the AD environment is the Domain Controllers OU. All other folders in the tree are containers. Containers can also contain objects. The Computers and Users containers are good examples of that. By default, any computer object will be stored in the Computers container. All the default user accounts and security groups are stored in the Users container. Similar to OUs, containers can also be used to delegate administration control. The only difference between containers and OUs is that group policies cannot apply to containers. Group policies can be assigned only to OUs. The system also does not allow you to create new containers other than containers created by the system.