To create an OU, perform the following steps:

1. Determine where you'd like this OU to live. If it lives directly underneath the root, then your root ID will be the parent. Alternatively, if this OU is going to be a child of another OU, use the ID of the OU instead. Obviously, if this is the first OU you're creating, the root will be the parent.

2. Use the CLI to create your OU, as follows:

        aws organizations create-organizational-unit 
          --parent-id <root-id or parent-ou-id> 
          --name <desired-ou-name>