If you don’t want to sync your entire Active Directory to Azure/Office 365 you can in the new AADSync set up a OU filter. Here is a short Checklist

1. Start Synchronization Service Manager
   1. Go to Connectors
   2. Select Properties on your AD Connector
   3. Select Configure Directory Partitions
   4. Click Containers (Enter Credentials)
   5. Select OUs to Sync
   6. Exit Properties
2. Initiate sync
   1. Right-click AD Connector and select Run – Full Import
   2. Right-click AD Connector and select Run – Delta Sync
   3. In Task Scheduler run the Azure AD Sync Scheduler or wait max of 3 hours
3. Verify in Azure AD/Office 365 that the sync was OK…

Links
http://andersonpatricio.ca/how-to-force-the-synchronization-using-aadsync/
http://oddytee.wordpress.com/2014/09/24/configure-filtering-with-aadsync/