If you are looking for a collection to query all the machines only from a particular OU not the SUB OU .Let say ,I have Top OU called Workstations\comp(120 systems) in eskon.net domain and in turn it has 3 sub OU's OU1(10 systems),OU2(20 systems),OU3(49 systems).
Here is the simple query to get machines only from TOP OU called workstations.
WHERE sms_r_system.systemouname = "eskon.net/workstations/comp" /*find all machines in that OU (but also finds machines in subOUs */
AND sms_r_system.resourceID NOT IN (SELECT resourceID FROM sms_r_system where systemouname LIKE "eskon.net/workstations/comp/%") /*remove records for machines in subOUs*/
The above collection should display only machines that are present in comp OU not from the SUB OU's (like OU1,OU2,OU3).