Okay, this is a little vague, so a screenshot of the rule you're editing would help.
However, I'm seeing two possibilities:
- You have a rule that is using a constant text value for the OU
- You have a rule that's using a Directory Service Group for the OU
If it's the former, I'm betting you'd have just edited the text. If it's the latter, you need to go to Build --> Groups and add a new Directory Service Group. This will pull in SECURITY GROUPS ONLY, though an OU can also be a Security Group. Once it's added, you'll be able to see it in the Rules editor under the "Directory Service Groups" drawer.