This is a short step by step guide how to make a dynamic group based on computers in a OU (Organizational Unit). 

1. From the Start Menu, select the Operations System Center Operations Console
2. In the Navigation pane (left), Click Authoring
3. In the Authoring pane, right-click Groups and choose New Group
4. In the Create Group Wizard – General Properties window, input a suitable name and choose a management pack, you should have one explicit management packs for all your overrides. Click Next
5. In the Create Group Wizard – Explicit Members window, choose any machines that will always be a member of this group even if they don’t fulfill the dynamic member formula. Click Next
6. In the Create Group Wizard – Dynamic Members window, click Create/Edit rules…
7. In the Create Group Wizard – Query Builder, choose Windows Computer from the drop-down menu and then click Add. Choose Organizational Unit in the property menu, equals in the operator menu and input a OU name in the Value field, for example OU=London,OU=Contoso,DC=contoso,DC=Local . Click OK
8. In the Create Group Wizard – Dynamic Members window, the formula will now look like this 
   ( Object is Windows Computer AND  ( Organizational Unit Equals OU=London,OU=Contoso,DC=Contoso,DC=Local ) )
9. In the Create Group Wizard – Dynamic Members window, click Next
10. In the Create Group Wizard – Subgroups window, choose subgroups to add to this group. Click Next
11. In the Create Group Wizard – Excluded Members, choose any machines that will not be a member of this group even if they don’t fulfill the dynamic member formula. Click Create

Please note that this will only include machines in the OU specified, it you want to include computers from another OU you can simple add a “OR” expression.