(In order to add students by OU the individual/user must be a Google Admin for the OU and an Admin on uPar license)
It may take a few minutes for the OU to be added. You will need to log out then back in to see the changes after the sync is complete.
*Seated accounts/licenses do not have the option to add students by Google OU. **Make sure you have any Teacher/Staff only OU unchecked.